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    [Transcriber's Note: A few typographical errors have been
    corrected. Details are given at the end of the text.]




HEADS OF LECTURES

ON A COURSE OF

EXPERIMENTAL PHILOSOPHY,

PARTICULARLY INCLUDING

CHEMISTRY,

DELIVERED AT THE NEW COLLEGE
IN HACKNEY.

_By JOSEPH PRIESTLEY, LL.D. F.R.S._

AC. IMP. PETROP. R. PARIS. HOLM. TAURIN. AUREL. MED.
PARIS. HARLEM. CANTAB. AMERIC. ET PHILAD. SOC.

QUI DOCET DISCIT.--WM. LILLY

LONDON:

PRINTED FOR J. JOHNSON, NO. 72, ST. PAUL'S CHURCH-YARD.

1794.




THE PREFACE.


Situated, as I happily am, in the neighbourhood of the _New College at
Hackney_, an institution that does honour to the Dissenters, an
institution open to all persons without distinction[1], and connected as
I am by friendship with the tutors, I was glad to give it every
assistance in my power; and therefore undertook to read the _Lectures on
History and General Policy_ which I had composed when I was tutor at
Warrington, and also to give another course on the subject of
_Experimental Philosophy_. With this view I drew up the following _Heads
of Lectures_; and, to save the students the trouble of transcribing
them, they are now printed. To other persons they may serve as a
compendious view of the most important discoveries relating to the
subject.

    [Footnote 1: One gentleman of the Roman Catholic persuasion, and
    several of the Church of England, are now in the College.]

As it was found most convenient, with respect to the other business of
the college, to confine this course to one lecture in a week, I
contrived to bring within that compass as much of the subject of
experimental philosophy as I well could, and especially to include the
whole of what is called _chemistry_, to which so much attention is now
given, and which presents so many new fields of philosophical
investigation.

Besides that the plan of the young gentlemen's studies would not admit
of it, I think it most advisable not to trouble beginners with more than
a large outline of any branch of science. By this means they are not
fatigued by too long an attention to any one subject, a greater variety
of articles may be brought before them, and in future life they may
pursue any of them as much farther as their inclination may dispose, and
their ability and opportunity shall enable, them to do it.

I do not give any account of the _experiments_ introduced into the
several lectures. They will be sufficiently indicated by the subjects of
them. They were as many as I could conveniently make within the time;
and where the experiments themselves could not be made, I usually
exhibited both the different substances employed in them, and those that
were the result of them.

As these lectures were calculated for the use of the students at the New
College, I prefix an _Address to them_, the same in substance with that
which I delivered to them at the close of the session of 1791. In it may
be seen a specimen of the language we hold to them on the subject of
_politics_, which with reasonable men will serve as an answer to the
many calumnies that have been thrown out against us, as disaffected to
the government of this country.

Such institutions will, indeed, always be objects of hatred and dread to
_bigots_ and the advocates for _arbitrary power_, but the pride of a
truly _free country_. I therefore conclude with my earnest prayer (the
accomplishment of which the present state of the College does not allow
us to doubt) ESTO PERPETUA.




THE DEDICATION.

TO THE STUDENTS AT THE NEW COLLEGE IN HACKNEY.


My young Friends,

Having drawn up the following _Heads of Lectures_ for your use, I take
the liberty thus publickly to dedicate them to you; and as I earnestly
wish for your improvement and happiness in all respects, excuse me if I
take the farther liberty of making a few observations, and giving you
some advice, of a more general nature, adapted to your age and
circumstances.

As you will soon leave this place of education, and enter upon your
several professions and employments, I hope your conduct will
demonstrate to the world the solid advantages of this institution, and
that the great expence attending it, and the best attention of the
managers, have not been bestowed in vain.

Many liberal friends of science, of virtue, and of religion, have
contributed to procure you the advantages which you enjoy. They have
spared no pains to provide able and careful tutors, and you have had
every other advantage for the prosecution of your studies that they
could procure you, unclogged by any subscription to articles of faith,
or obligation of any other kind, besides such as they have deemed
necessary for your own good, and to give the institution its greatest
effect. This is an advantage you could not have found elsewhere, at
least in this country. And in every seminary of education much more
depends upon opportunity for study, free from any obstruction, and undue
bias, than upon the ability of tutors; though there is an additional
advantage when they are able men, and eminent in the branches of science
which they undertake to teach. But this is by no means so essential as
many other circumstances.

Whatever be the qualifications of your tutors, your improvement must
chiefly depend upon yourselves. They cannot _think_ or _labour_ for
you. They can only put you in the best way of thinking and labouring for
yourselves. If, therefore, you get knowledge, you must acquire it by
your own industry. You must form all conclusions, and all maxims, for
yourselves, from premises and _data_ collected, and considered by
yourselves. And it is the great object of this institution to remove
every bias the mind can lie under, and give the greatest scope to true
freedom of thinking and inquiry. And provided you be intelligent and
virtuous men, and good citizens, it will be no cause of regret to the
friends of this institution, if, with respect to _religion_, or
_politics_, you adopt systems of principles, and maxims of conduct, very
different from theirs.

Give me leave, now that I am addressing you as _young men_, and young
_students_, to suggest a caution on a subject, of the importance of
which it is hardly possible that you should be sufficiently aware,
because it is only impressed by that _experience_ which you have not yet
had. I mean that degree of vanity which generally accompanies the
acquirements that diligent persons of your age make in places of liberal
education, and the contempt they are too apt to entertain for those who
have not made the same proficiency with themselves. And I assure you,
that in the observations I shall make on this subject, I have no view
whatever to any thing that I have observed, or heard, of any of you in
particular. But I have been in your situation myself, and I know the
importance of these observations to students in general.

You are now at an age in which young persons usually make the most
sensible advances in knowledge, and in which the understanding appears
to ripen the most rapid manner. You are able to say every year, every
month, and almost every day, what particular advances you have made, and
how much you know more than you did before. And being taught, and
accustomed, to express your thoughts in writing, you find yourselves
qualified to do this in a manner of which you had no idea, or
expectation, but a little time ago. You also perfectly remember what you
have so recently learned, and many respects may be more particular and
exact than even your tutors themselves.

The almost unavoidable effect of this is a high idea of your own powers
and attainments, and too often a proportionable contempt of those who,
not having had equal advantages, cannot do what you are easily capable
of. A certain degree of vanity is, therefore, excusable in young
persons; and, indeed, it is by means of it that they are excited to
exert themselves in a manner that they would not otherwise have done.
But be careful that this temper be not indulged to excess, for it will
then be found to have serious ill consequences; the least of which is
the precluding future improvement, from being already satisfied with
yourselves, and conscious of a sufficient superiority over others.

The foundation of this self-conceit, on account of literary attainments,
will be found to be extremely weak. In fact, we learn more before the
period to which you are now arrived, and I hope you will continue to
learn more after it, without its being so much noticed; and the
_ability_ that is discovered in the acquirements which are the subjects
of this vanity is not greater than appears on other occasions. Only they
are not so conspicuous.

What we all learn in the first three years of our lives, is much more
extraordinary in its nature than all that we acquire afterwards. I mean
the perfect use of our limbs, and the elements of speech. What we learn
in a month in that early period of life, could not, if we were brought
up in the ignorance of it, be learned in a year at any subsequent
period. But these acquirements being universal, and what the
circumstances in which we are all necessarily in compel us to learn, it
does not appear extraordinary in any particular individual. Also, the
proficiency that boys make at a grammar school, in which, in general,
the dead languages only are taught (a knowledge of which is commonly the
result of severe application) is too common to be the cause of much
self-conceit. But the advances that are made at places of liberal
education are both less common, and of a more conspicuous nature.

You will also find, if you continue your application to study, that it
is only the elements of science that you can acquire here, and that if
you live many years, they will bear but a small proportion to your
future acquirements. But those future acquirements, in consequence of
their bearing a less proportion to your whole stock of knowledge, will
not be so conspicuous. Thus, though all the buildings that in one year
are added to such a city as London would make a pretty large country
town, they bear so small a proportion to what was built before, that
they are not much noticed; whereas, had half the quantity of building
been erected in a place where no house had existed before, it would have
been a memorable event in the history of the country.

Also, as in old cities many buildings will fall to decay, while new ones
are added; you must expect to forget much of what you now know. No man
can give equal attention to every object; and as we advance in life,
we, in general, only learn new things at the expence of the old ones.
But then they are the more valuable articles of knowledge, the more
general and leading principles, which remain with us; while the more
useless ones, things to which we give less attention because we find
them to be of less use, disappear. Yet it is no uncommon thing for
ingenious students to despise old scholars who are not so ready in the
_minutiae_ of literature, though they may have forgotten more than those
youths ever knew, and may retain what they cannot acquire without
forgetting as much.

Another observation proper to lessen the conceit of literary men, is,
that genius is not confined to _them_, but is equally great, though not
equally conspicuous, in every other line of life, and especially in
manufactures and the arts. Here, however, discoveries equal, with
respect to _sagacity_, to those of Newton, contribute little to
posthumous fame, because the discoverers are not writers.

But the greatest branch of intellectual excellence with respect to
which every other is nothing, and which, from its nature, can never be
foundation of any self-conceit, is _virtue_, or right dispositions of
mind, leading to right conduct in life. Proper sentiments, and just
affections of mind, arise from just, and often comprehensive, views of
things, past, present, and to come. And if the real greatness of any
thought, or action, be estimated by the number of elements that
constitute it, and its remoteness from the dictates of sense and natural
appetite, a virtuous and pious man will appear to be a much more
dignified character, more proper to be viewed with admiration and
esteem, than the greatest scholar; discovering, in fact, greater
comprehension and force of mind. I mean, however, that virtue which is
the result of reflection, of discipline, and much voluntary exertion,
which, though operating with equal promptness and facility, is as much
superior to mere _innocence_, and what is commonly called _good nature_,
as motions secondarily automative are to those that are primarily so; a
comparison which you who have studied _Hartley's Theory of the Mind_
will see the force of.

These considerations I take the liberty to suggest, as being proper to
lessen that vanity which is so incident to those who distinguish
themselves in the fields of literature, and which, operating like the
acquisition of riches, or power, or any possession that is _rare_ among
men, instead of enlarging, may tend to contract the mind, by confining
its attention to itself. Beginning with a generous emulation, it
proceeds to envy and jealousy, and ends in actual hatred and malignity,
against which you cannot, surely, be too strongly put upon your guard;
this being the greatest depravity to which human nature is subject, and
which yet, like any other vice, may be in full possession of the mind,
without the person himself knowing, or suspecting it; unless he give
more attention to his feelings than most persons do. If no man ever
thought himself to be avaricious, or cruel, can it be expected that any
person should ever discover that he is too self-conceited?

Better, however, infinitely better, were it to rank with the merest
dunces, than have the conceit and malignity (produced originally from
conceit) of some who have distinguished themselves the most as
linguists, critics, and poets. Even the study of nature, though, from
its vast extent, it is less apt to produce this baneful effect, is not
always a sufficient guard against it. This is an affecting and an
alarming consideration. But in the intellectual, as in the civil and
commercial world, we gain nothing but at the risk of some loss; and in
this case the possible gain is worth the risk of even this great loss.

For when literary, and scientific excellence coincide with that which is
of a moral nature, it adds unspeakably to the value of a character.
Ingenuity coupled with modesty, and great genius with benevolence and
true piety, constitute the perfection of human character, and is what we
should ever have in view. And a course of education in which both these
objects are equally attended to, is the only one that deserves to be
called _liberal_: but such as, I hope, you have found this to be.

Give me leave further to observe to you, that the time that you spend in
a place of liberal education is of more importance to you than you can
be at present aware of. Whatever be the sphere of life for which you are
destined, the probability is, that you will hereafter have but little
leisure for reading and studying, compared to what you have now.
Besides, general maxims of all kinds, such as are the foundation of all
our future conduct, in morals, religion, or politics, are generally
formed at your time of life. From this period expect no great change in
your opinions, or conduct; because now it is that you give particular
attention to the forming your opinions on all subjects of importance; so
that very little that is materially new to you can be expected to occur
to you in future life, and almost every thing that you would choose to
read will only tend to confirm you in the general principles that you
will now adopt. There are, no doubt, exceptions to this, as well as
every other general observation; but it is wisdom to suppose, and to act
upon the supposition, that we are constituted as the generality of
mankind are, and that we shall feel, and act, as they do. Since, then,
so much depends on the leading principles and maxims which you will now
adopt, be it your care to form just and good ones, and let no authority
of tutors, or others, have any undue influence over you. In all cases
think and judge for yourselves, and especially on all subjects of
importance, and with as much attention as you can give to them.

It may not be amiss, in the present state of things, to say something
respecting another subject, which now commands universal attention. You
cannot but be apprised, that many persons entertain a prejudice against
this College, on account of the republican, and, as they choose to call
them, the licentious, principles of government, which are supposed to be
taught here. Show, then, by your general conversation, and conduct, that
you are the friends of peace and good order; and that, whatever may be
your opinions with respect to the best form of government for people who
have no previous prejudices or habits, you will do every thing in your
power for the preservation of that form of it which the generality of
your countrymen approve, and under which you live, which is all that can
be reasonably expected of any subject. As it is not necessary that every
good son should think his parent the wisest and best man in the world,
but it is thought sufficient if the son pay due respect and obedience to
his parent; so neither is it to be expected that every man should be of
opinion that the form of government under which he happens to be born is
the best of all possible forms of government. It is enough that he
submit to it, and that he make no attempt to bring about any change,
except by fair reasoning, and endeavouring to convince his countrymen,
that it is in their power to better their condition in that respect, as
well as in any other. Think, therefore, speak, and write, with the
greatest freedom on the subject of government, particular or general,
as well as on any other that may come before you. It can only be avowed
tyranny that would prevent this. But at the same time submit yourselves,
and promote submission in others, to that form of government which you
find to be most approved, in this country, which at present
unquestionably is that by _King, Lords, and Commons_.

As to _religion_, we may, surely, be allowed to think and act entirely
for ourselves; in all cases obeying God and conscience rather than man.
But let us be thankful for the degree of liberty that we are allowed,
though it be not all that we are justly entitled to; and let us not use
any other means than reason and argument in order to better our
condition. By this peaceable and steady conduct we shall at length
convince those who will hear reason, that we are entitled to greater
consideration; and doubt not but whatever is _true_ and _right_, will
finally prevail, and be universally established.

That any of your tutors, or any of the friends of this institution,
wish to promote reformation, in church or state, by any other means than
those of reason, and argument, is a _calumny_, utterly void of
foundation, or probability. But your conduct, dispersed as you will soon
be in different parts of the country, will be the best means of refuting
it. Let us leave the method of proceeding by _riot_ and _tumult_ to
those persons to whose schemes such proceedings are congenial. Truth
stands in no need of such support, and will always triumph when assailed
by such weapons. In return, then, for the advantages which you have
enjoyed in this institution, do it this service; and in recommending it,
I trust you are doing substantial service to the cause of liberty and
truth; and conferring a most important benefit on your country, and on
mankind.




CONTENTS.


LECTURE I.
_The Introduction_                                              1

LECTURE II.
_Of the Properties of all Matter_                               9
_Of Aeriform Substances_                                       13

LECTURE III.
_Of Atmospherical Air_                                         15

LECTURE IV.
_Of Dephlogisticated Air_                                      18
_Of Phlogisticated Air_                                        20

LECTURE V.
_Of Inflammable Air_                                           21

LECTURE VI.
_Of Nitrous Air_                                               23

LECTURE VII.
_Of Fixed Air_                                                 27
_Of Hepatic Air_                                               29
_Of Phosphoric Air_                                            30

LECTURE VIII.
_Of Dephlogisticated Marine Acid Air_                          31
_Of Phlogisticated Marine Acid Air_                            32

LECTURE IX.
_Of Vitriolic Acid Air_                                        34
_Of Fluor Acid Air_                                            36

LECTURE X.
_Of Alkaline Air_                                              37
_Miscellaneous Observations relating to Air_                   38

LECTURE XI.
_Of Liquid Substances; and first of Water_                     41

LECTURE XII.
_Of the Nitrous Acid_                                          45

LECTURE XIII.
_Of the Vitriolic Acid_                                        49
_Of the Marine Acid_                                           52

LECTURE XIV.
_Of the Vegetable Acids, and others of a less perfect nature_  54

LECTURE XV.
_Of the Phosphoric Acid_                                       59

LECTURE XVI.
_Of Alkalis_                                                   62

LECTURE XVII.
_Of Liquid Inflammable Substances_                             66
_Of AEther_                                                     68

LECTURE XVIII.
_Of Oil_                                                       70

LECTURE XIX.
_Of Solid Substances_                                          76
_Of Calcareous Earth_                                          77
_Of Siliceous Earth_                                           79

LECTURE XX.
_Of Argillaceous Earth_                                        81
_Of Terra Ponderosa_                                           82
_Of Magnesia_                                                  83

LECTURE XXI.
_Of Ores_                                                      84
_Of Gold_                                                      87

LECTURE XXII.
_Of Silver_                                                    90
_Of Platina_                                                   92

LECTURE XXIII.
_Of Mercury_                                                   94

LECTURE XXIV.
_Of Lead_                                                      99
_Of Copper_                                                   101

LECTURE XXV.
_Of Iron_                                                     105

LECTURE XXVI.
_Of Tin_                                                      109
_Of the Semi-metals_                                          113

LECTURE XXVII.
_Of Nickel_                                                   115
_Of Arsenic_                                                  116
_Of Cobalt_                                                   118
_Of Zinc_                                                     119

LECTURE XXVIII.
_Of Antimony_                                                 121
_Of Manganese_                                                123
_Of Wolfram_                                                  124
_Of Molybdena_                                                125
_Of Solid Combustible Substances_                             126

LECTURE XXIX.
_Of the Doctrine of Phlogiston and the Composition of Water_  127

LECTURE XXX.
_Of Heat_                                                     135

LECTURE XXXI.
_Of Animal Heat_                                              145

LECTURE XXXII.
_Of Light_                                                    148

LECTURE XXXIII.
_Of Magnetism_                                                155

LECTURE XXXIV.
_Of Electricity_                                              162

LECTURE XXXV.
_The same Subject continued_                                  170

LECTURE XXXVI.
_The same Subject continued_                                  177




LECTURES ON EXPERIMENTAL PHILOSOPHY.




LECTURE I.


_The Introduction._

The object of experimental philosophy is the knowledge of nature in
general, or more strictly, that of the properties of natural substances,
and of the changes of those properties in different circumstances. This
knowledge can only be attained by _experiment_, or _observation_; as
that clay is capable of becoming hard by means of fire, and thereby
being made into bricks, and that by the same means lime-stone can be
converted into quick-lime, and by the addition of water and sand, make
mortar. It is by observation also that we discover that stones and other
heavy bodies fall to the ground, and that a magnet will attract iron. In
other words, experimental philosophy is an investigation of the wisdom
of God in the works and _the laws of nature_, so that it is one of the
greatest objects to the mind of man, and opens a field of inquiry which
has no bounds; every advance we make suggesting new doubts and subjects
of farther inquiry.

The uniformity we discover in the properties of natural substances
enables us to lay down general rules, or principles, which, being
invariable, we call the laws of nature; and by our knowledge of these
laws we are able to predict, and at our own pleasure to produce,
particular results, and this is the source of all the powers of man. It
is the direction we acquire of the powers of nature; so that, as Lord
Bacon observed, _knowledge is power_.

All arts and manufactures are derived from science. Thus the doctrine of
_mechanics_ is an application of the law of gravitation. Every thing we
are capable of doing by means of the steam-engine is derived from our
knowledge of the properties of water in steam; all the great effects of
gunpowder we owe to our knowledge of the composition, and chemical
properties, of that substance.

Every new appearance in nature is preceded by some new circumstance, and
to this, or rather to something always attending it, we say that the
appearance is _owing_. This circumstance we therefore call the _cause_,
and the new appearance the _effect_ of that cause. Thus we say that the
union of phlogiston to a particular kind of earth is the cause of its
becoming a metal.

It is one of the principal rules of philosophizing to admit no more
causes than are necessary to account for the effects. Thus, if the power
of gravity, by which heavy bodies fall to the earth, be sufficient to
retain the planets in their orbits, we are authorized to reject the
_Cartesian Vortices_. In other words, we must make no more general
propositions than are necessary to comprehend all the particulars
contained in them. Thus, after having observed that iron consists of a
particular kind of earth united to phlogiston, and that it is soluble in
acids; and that the same is true of all other metallic substances, we
say, universally, that all metals consist of a peculiar earth and
phlogiston, and that they are all soluble in some acid.

Of the circumstances which occasion a change in the properties of
bodies, some are the addition of what are properly called _substances_,
or things that are the objects of our senses, being _visible_,
_tangible_, or having _weight_, &c. Thus the addition of an acid changes
an alkali into a neutral salt. But other changes are occasioned either
by a change of texture in the substance itself, or the addition of
something that is not the object of any of our senses. Thus, a piece of
steel becomes a magnet by the touch of another magnet, and a drop of
glass acquires the property of flying asunder by a small fracture, in
consequence of falling when red hot into cold water. Such also, in the
opinion of some, is the difference between hot and cold substances.

Till the nature of the cause be ascertained, it is convenient to make
use of the term _principle_, as including both of the above-mentioned
causes of the change of properties in bodies. Thus, whatever be the real
cause of gravity, or of inflammability, we may speak of the _principle
of gravity_, or of _inflammability_; whether, with Newton, we suppose
gravity to be occasioned by a fluid pervading the whole universe, which
he termed _aether_, and whether inflammability be caused by the presence
of a real substance called phlogiston, or not. In this manner we use the
letters _x_ and _y_ to denote unknown quantities in algebra.

When changes are made in substances by the addition of other substances,
they make what is called a _chemical union_; and in this case the
properties of the compound cannot with any certainty be deduced from
those of the component parts, but must be ascertained by fresh
experiments. Thus, from the specific gravities, or the degrees of
fusibility, of two metals, those of the compound cannot be predicted.
Neither water nor acid of vitriol will separately dissolve iron, so as
to produce inflammable air, but both together will do it. However, the
properties of similar compounds are similar to one another. Thus, all
metals dissolved in acids are precipitated by mild alkalis. This
chemical union of two substances we ascribe to a certain _elective
attraction_, or _affinity_ that subsists between them, in consequence of
which they unite with one another whenever a proper opportunity offers,
in preference to those substances to which they were before united. Thus
the vitriolic acid, having a stronger affinity with the vegetable alkali
which is the basis of nitre, will unite with that alkali, and with it
form another compound, called _vitriolated tartar_, while the acid of
nitre, being detached from its base, is collected separately.

When two substances compose one liquid, and a third, which has a
stronger affinity with either of the two parts than they have with each
other, is added to them, it will unite with that part, and take its
place in the solution, while the other will in many cases be
precipitated, and may be collected. Thus the earth of alum is
precipitated from a solution of alum by salt of tartar. This is the case
of _simple affinity_.

When both the substances are compounds, the component parts of which
have a weaker affinity with each other than they have with those of the
other compound, two new combinations are formed, and this is called a
case of _double affinity_. Thus when phlogisticated alkali is poured
into a solution of green vitriol, the acid of the vitriol unites with
the alkali, while the phlogiston joining the calx of iron makes Prussian
blue.

All nature lying open to our investigation, we must consider the
different parts in some order. But it is not very material which we
adopt, because, begin where we will, the properties of the substances we
first treat of will be connected with those which must be particularly
considered afterwards, the changes in one substance being occasioned by
its union with another. It will be impossible, for example, to explain
the properties of metals without considering the _acids_, because by
their union with acids very important changes are made in their
properties.

There have been three principal methods of arranging natural substances.
One is according to the _three kingdoms_, as they are called, into which
they have been distributed, viz. the _mineral_, the _vegetable_, and the
_animal_. Another is according to the _elements_ which enter into their
composition, and a third according to the _form_ in which they are
usually found, viz. _aerial_, _fluid_, or _solid_. Upon the whole this
last appears to me to be the most convenient, especially as it is easy
to intermix general observations concerning the other divisions when
they are particularly wanted. As there will be frequent occasion to
speak of the component and elementary parts of all substances, I shall
here observe, that, according to the latest observations, the following
appear to be the elements which compose all natural substances, viz.
_dephlogisticated air_, or the _acidifying principle_; _phlogiston_, or
the _alkaline principle_; the different _earths_, and the principles of
_heat_, _light_, and _electricity_. Besides these, there are the
following principles which have not been proved to be substances, viz.
_attraction_, _repulsion_, and _magnetism_. By the help of these
principles we are able, according to the present state of natural
knowledge, to explain all the appearances that have yet occurred to us.




LECTURE II.


_Of the Properties of all Matter._

Before I consider the properties of particular substances, it will be
proper to mention those which are common to them all. But I shall first
observe, that the term _substance_ has no proper idea annexed to it, but
is merely a convenience in speech; since we cannot speak of the
properties of substances, such as _hard_, _round_, _coloured_, &c. &c.
(which circumstances alone affect our senses, and thereby give proper
ideas) without saying that they inhere in, or belong to, some _thing_,
_substance_, or _substratum_. The terms _being_ and _person_ are also in
the same predicament.

One property of all substances is _extension_, since they all occupy
some portion of space.

The incapacity of any substance to change its place has been termed,
though improperly, the _vis inertiae_ of matter. It is sufficient to say,
that neither this, nor any other effect can be produced without a cause.

_Infinite divisibility_ is a necessary property of all extended
substance; and from this circumstance it will follow, that the smallest
quantity of solid matter may be made to fill the largest space, and yet
none of the pores shall exceed the smallest given magnitude; and
consequently, that, for any thing we know to the contrary, all the
bodies in the universe may be comprized in the smallest space.

Another property usually ascribed to all matter is _impenetrability_,
or the necessary exclusion of any substance from the place occupied by
another. But the only proof of impenetrability is the _resistance_ that
we find to our endeavours to put one substance into the place of
another; and it is demonstrated by experiments, that this resistance is
not occasioned by the actual contact of the substances, but by a power
of repulsion acting at a real distance from their surfaces. It requires
a considerable force to bring two solid substances into as near contact
as the particles of the same substance; and that _these_ are not in
actual contact is evident, from their being capable of being brought
nearer by cold; and this is most remarkable with respect to the
heaviest, that is, the densest, of all substances, viz. the metals.

A more positive argument for the penetrability of matter is, that the
particles of light, after entering the densest transparent substance, do
not appear to meet with any obstruction to their progress till they come
to the opposite side.

The powers of _attraction_ and _repulsion_ seem to be common to all
matter, and the component parts of all substances are kept in their
places by the due balance of those opposite powers. If, by any means,
the particles of any substance be removed beyond their sphere of mutual
attraction, they repel one another, as those of water when it becomes
steam.

Of the different kinds of attraction, that of _gravitation_ seems to
extend to the greatest possible distance; but that which keeps together
the parts of the same substance, thence called the _attraction of
cohesion_, and the different kinds of chemical attractions, called
_affinities_, only act at a small distance. Of the causes of these
attractions we are entirely ignorant.


_Of Aeriform Substances._

Aeriform substances, of which the air that we breathe is one, though
invisible, are real substances, as appears by their excluding other
substances.

That the air has _weight_ appears by actually weighing a vessel before
and after it is exhausted of air by means of an air-pump (an instrument
contrived for that purpose) by its bursting a bladder, and various other
experiments.

Air, being a fluid, presses in all directions, as in the experiment of
the fountain in _vacuo_, and others.

The weight of the air is the cause of the suspension of mercury in a
barometer, and of the action of pumps. The weight of atmospherical air
is to that of water in the proportion of about 1 to 800, so as to press
with the weight of about fourteen pounds on every square inch of
surface.

Air, being an elastic fluid, is capable of occupying more or less space
according to the pressure which it sustains, as appears by a bladder
partially filled with air being expanded when the air is drawn from a
receiver in which it is put, by means of the air-pump, and compressed in
the condensing engine, an instrument the reverse of the air-pump.

Air is necessary to the conveyance of sound, to the existence of flame,
and to animal life.




LECTURE III.


_Of Atmospherical Air._

The first species of air that offers itself to our consideration is that
of the atmosphere, which appears to consist of a mixture of two kinds of
air, of different and opposite qualities, viz., dephlogisticated and
phlogisticated, in the proportion of about one third of the former to
two thirds of the latter. It is by means of the former of these two
ingredients that it is capable of supporting flame and animal life.

This composition of atmospherical air is proved by several substances
absorbing the dephlogisticated air, and leaving the phlogisticated. All
these processes have been termed _phlogistic_, because the effect is not
produced but by substances supposed to contain phlogiston in a volatile
state; and by the affinity between phlogiston and the dephlogisticated
part of the air, the one is separated from the other. Of these processes
are the calcination of metals, a mixture of iron-filings and sulphur,
liver of sulphur, the burning of phosphorus, and the effluvia of
flowers.

In some cases, however, it is not so clear that any thing is emitted
from the substance that produces this effect; for water deprived of all
air will absorb the dephlogisticated part of the atmospherical in
preference to the phlogisticated part.

As the purity of atmospherical air, or its fitness for respiration,
depends upon the proportion of the dephlogisticated air that it
contains, any of the above-mentioned processes will suffice to determine
it. The more any given quantity of air is diminished by any of them, the
purer it was before the diminution. But this effect is produced the most
quickly by a mixture of nitrous air, or firing inflammable air in it,
being almost instantaneous.

In order to measure the purity of air, it is convenient to take more of
the nitrous or inflammable air than is necessary to saturate the
dephlogisticated air it contains. Equal quantities of each best answer
the purpose. Supposing a given quantity of atmospherical air to be mixed
with an equal quantity of nitrous air, and the residuum to be 1.1
measure, the proportion of dephlogisticated and phlogisticated air in it
may be found by the following arithmetical operation, it being here
taken for granted that one measure of pure dephlogisticated air will
saturate two measures of pure nitrous air.

      2.0 viz. one of each.
      1.1 the residuum.
     -----
    3)0.9 the quantity that has disappeared.
      0.3 the dephlogisticated air contained
            in the measure of the air examined.

And this substracted from 1 leaves .7 for the proportion of
phlogisticated air in it.




LECTURE IV.


_Of Dephlogisticated Air._

Dephlogisticated air, which is one of the component parts of
atmospherical air, is a principal element in the composition of acids,
and may be extracted by means of heat from many substances which contain
them, especially the nitrous and vitriolic; as from nitre, red
precipitate, the vitriols, and turbith mineral, and also from these two
acids themselves, exposed to a red heat in an earthen tube. This kind of
air is also contained in several substances which had attracted it from
the atmosphere, as from precipitate _per se_, _minium_, & _manganese_.

Dephlogisticated air is likewise produced by the action of light upon
green vegetables; and this seems to be the chief means employed by
nature to preserve the purity of the atmosphere.

It is this ingredient in atmospheric air that enables it to support
combustion and animal life. By means of it the most intense heat may be
produced, and in the purest of it animals will live nearly five times as
long as in an equal quantity of atmospherical air.

In respiration, part of this air, passing the membrane of the lungs,
unites with the blood, and imparts to it its florid colour, while the
remainder, uniting with phlogiston exhaled from the venous blood, forms
fixed air. It is dephlogisticated air combined with water that enables
fishes to live in it.

Dephlogisticated air is something heavier than atmospherical air, and
the purity of it measured by mixing with it two equal quantities of
nitrous or inflammable air, deducing the residuum after the diminution
from the three measures employed, and dividing the remainder by 3, as in
the process for common air.


_Of Phlogisticated Air._

The other ingredient in the composition of atmospherical air is
phlogisticated air. It is procured by extracting the dephlogisticated
part of the common air, as by the calcination of metals, &c. &c. by
dissolving animal substances in nitrous acid, and also by the union of
phlogiston with nitrous air, as by heating iron in it, and by a mixture
of iron-filings and sulphur.

Phlogisticated air extinguishes a candle, is entirely unfit for
respiration, and is something lighter than common air. It is not capable
of decomposition, except by exploding it together with a superabundance
of dephlogisticated air, and a quantity of inflammable air, or by taking
the electric spark repeatedly in a mixture of it and dephlogisticated
air. In these cases nitrous acid is formed.




LECTURE V.


_Of Inflammable Air._

Inflammable air is procured from all combustible substances by means of
heat and water, and from several of the metals, especially iron, zink,
and tin, by the vitriolic and marine acids.

From oils and spirit of wine it is procured by the electric spark. By
the same means also alkaline air is converted into it.

That which is procured from metals, especially by steam, is the purest
and the lightest, about ten times lighter than common air; in
consequence of which, if a sufficient quantity be confined in a light
covering, it is possible to make it carry up heavy weights.

When it is procured from animal or vegetable substances, it is of a
heavier kind, and burns with a lambent flame, of various colours,
according to the circumstances.

Calces of metals heated in inflammable air are revived, and the air
absorbed; and since all the metals are revived in the same inflammable
air, the principle of metallization, or _phlogiston_, appears to be the
same in them all.

Pure inflammable air seems to consist of phlogiston and water, and the
lambent kinds to be the same thing, with the addition of some oily
vapour diffused through it.




LECTURE VI.


_Of Nitrous Air._

Nitrous air is procured by dissolving most of the metals, especially
iron, mercury, and copper, in the nitrous acid; but that from mercury
seems to be the purest. Nitrous air produced from copper contains a
mixture of phlogisticated air. Some nitrous air is also obtained from
the solution of all vegetable substances in nitrous acid; whereas animal
substances in the same process yield chiefly phlogisticated air: but in
both these cases there is a mixture of fixed air.

This species of air is likewise produced by impregnating water with
nitrous vapour. This process continues to have this effect after the
water becomes blue, but ceases when it turns green; there not then,
probably, being a sufficient proportion of water. Nitrous air is
likewise produced by volatile alkali passing over red hot manganese, or
green vitriol, when they are yielding dephlogisticated air. This shews
that dephlogisticated air is one ingredient in the composition of
nitrous air, and the same thing appears by pyrophorus burning in it. On
the contrary, when nitrous air is made to pass over red-hot iron,
volatile alkali is produced.

Nitrous air is completely decomposed by a mixture of about half its bulk
of dephlogisticated air, and the produce is nitrous acid. And as nitrous
acid is likewise formed by the union of inflammable and dephlogisticated
air, one principal ingredient in nitrous air must be common to it and
inflammable air, or phlogiston. This air is likewise decomposed by
dephlogisticated nitrous acid, which by this means becomes
phlogisticated. It is also decomposed by a solution of green vitriol,
which by this means becomes black, and when exposed to the air, or
heated, emits nitrous air, and recovers its former colour. These
decompositions of nitrous air seem to be effected by depriving it of
phlogiston, and thereby reducing it to the phlogisticated air originally
contained in it.

This kind of air is diminished to about one fourth of its bulk by a
mixture of iron filings and brimstone, or by heating iron in it, or
calcining other metals in it, when the remainder is phlogisticated air.
All that iron gets in this process is an addition of weight, which
appears to be water, but it loses its phlogiston, so that nitrous air
seems to contain more phlogiston, and less water than phlogisticated
air.

Nitrous air and dephlogisticated air will act upon one another through a
bladder, but in this case there remains about one-fourth of the bulk of
nitrous air, and that is phlogisticated air; so that in this case there
seems to be a conversion of nitrous air into phlogisticated air without
any addition of phlogiston.

Nitrous air is decomposed by pyrophorus, and by agitation in olive oil,
which becomes coagulated by the process. It is also absorbed by spirit
of turpentine, by ether, by spirit of wine, and alkaline liquors.

It is imbibed by charcoal, and both that air which is afterwards
expelled from it by heat, and that which remains unabsorbed, is
phlogisticated air.

Nitrous air resists putrefaction, but is diminished by the animal
substances exposed to it to about a fourth of its bulk, and becomes
phlogisticated air. It is likewise fatal to plants, and particularly to
insects.

When nitrous air is long exposed to iron, it is diminished and brought
into a state in which a candle will burn in it, though no animal can
breathe it. But this peculiar modification of nitrous air, called
_dephlogisticated nitrous air_, is produced with the greatest certainty
by dissolving iron in spirit of nitre saturated with copper,
impregnating water with this air, and then expelling it from the water
by heat. If bits of earthen ware be heated in this dephlogisticated
nitrous air, a great proportion of it becomes permanent air, not
miscible with water, and nearly as pure as common air, so that the
principle of _heat_ seems to be wanting to constitute it permanent air.




LECTURE VII.


_Of Fixed Air._

Having considered the properties of those kinds of air which are not
readily absorbed by water, and therefore may be confined by it, I
proceed to those which _are_ absorbed by it, and which require to be
confined by mercury. There are two kinds, however, in a middle state
between these, being absorbed by water, but not very readily; a
considerable time, or agitation, being necessary for that purpose. The
first of these is _fixed air_.

This kind of air is obtained in the purest state by dissolving marble,
lime-stone, and other kinds of mild calcareous earth in any acid. It is
also obtained by the burning, or the putrefaction, of both animal and
vegetable substances, but with a mixture of both phlogisticated and
inflammable air. Fixed air is also produced by heating together iron
filings and red precipitate; the former of which would alone yield
inflammable air, and the latter dephlogisticated. Fixed air is therefore
a combination of these two kinds of air.

Another fact which proves the same thing is, that if charcoal of copper
be heated in dephlogisticated air, almost the whole of it will be
converted into fixed air. On the same principle fixed air is produced
when iron, and other inflammable substances, are burned in
dephlogisticated air, and also when minium, and other substances
containing dephlogisticated air, are heated in inflammable air.

That water is an essential part of fixed air is proved by an experiment
upon _terra ponderosa aerata_, which yields fixed air when it is
dissolved in an acid, but not by mere heat. If steam, however, be
admitted to it in that state, it will yield as much fixed air as when it
is dissolved in an acid.

Water absorbs something more than its own bulk of fixed air, and then
becomes a proper acid. Iron dissolved in this water makes it a proper
chalybeate; as without iron it is of the same nature with Pyrmont or
Seltzer water, which by this means may be made artificially.

Ice will not imbibe this air, and therefore freezing expels it from
water.

Fixed air extinguishes flame, and is fatal to animals breathing in it.
Also water impregnated with this air is fatal to fishes, and highly
injurious to plants. But water thus impregnated will prevent, in a great
measure, the putrefaction of animal substances.

Fixed air thrown into the intestines, by way of glyster, has been found
to give relief in some cases of putrid disease.


_Of Hepatic Air._

Another species of air absorbed by water, but not instantly, is termed
_hepatic air_, being produced by the solution of liver of sulphur, or of
sulphurated iron, in vitriolic or marine acid.

Water imbibes about twice its bulk of this kind of air, and it is then
the same thing with the sulphureous waters of Harrowgate.


_Of Phosphoric Air._

Phosphoric air is produced by the solution of phosphorus in caustic
fixed alkali. If this air be confined by mercury, it will take fire on
being admitted to atmospheric, and much more to dephlogisticated air.
After agitation in water it loses this property, and the residuum is
merely inflammable air, with no great diminution of its bulk. This kind
of air, therefore, probably consists of phosphorus dissolved in
inflammable air; though it cannot be made by melting it in inflammable
air.




LECTURE VIII.


_Of Dephlogisticated Marine Acid Air._

This species of air is produced by heating spirit of salt with
manganese; or more readily, by pouring acid of vitriol on a mixture of
salt and manganese, in the proportion of about 16 of the former to 6 of
the latter. In this case the acid of vitriol decomposes the salt, and
the marine acid, disengaged in the form of air, takes dephlogisticated
air from the manganese; so that this species of air seems to consist of
marine acid vapour, and dephlogisticated air.

This species of air has a peculiarly pungent smell, and is absorbed by
water as readily as fixed air.

The water takes about twice its bulk of it; and thereby acquires a
yellowish tinge. Both this air, and the water impregnated with it,
discharges vegetable colours from linen or cotton, and is thereby
useful in bleaching.

This air when cold coagulates into a yellowish substance. It dissolves
mercury, and with it forms _corrosive sublimate_.


_Of Phlogisticated Marine Acid Air._

Besides the preceding kinds of air which are slowly absorbed by water,
there are others which are absorbed by it very rapidly, so that they
cannot be confined but by mercury.

Of this kind is _phlogisticated marine acid air_, procured by the acid
of vitriol and common salt; the former seizing upon the alkaline basis
of the latter, and thereby expelling the marine acid in the form of air.

It is called _phlogisticated_ to distinguish it from _dephlogisticated
marine acid air_, which seems to be the same thing, with the addition of
dephlogisticated air.

Phlogisticated marine acid air is heavier than common air. It
extinguishes a candle with a blue flame. It dissolves many substances
containing phlogiston, as iron, dry flesh, &c. and thereby forms a
little inflammable air. Water absorbs 360 times its bulk of this air,
and is then the strongest spirit of salt. It absorbs one-sixth more than
its bulk of alkaline air, and with it forms the common sal ammoniac. Its
affinity to water enables it to dissolve ice, and to deprive borax,
nitre, and other saline substances, of the water that enters into their
composition.




LECTURE IX.


_Of Vitriolic Acid Air._

Vitriolic acid air is procured by heating in hot acid of vitriol almost
any substance containing phlogiston, especially the metals which are
soluble in that acid, as copper, mercury, &c. This kind of air is
heavier than common air, and extinguishes a candle, but without any
particular colour of its flame. It will not dislodge the nitrous or
marine acids from any substance containing them.

By its affinity to water it deprives borax of it.

One measure of this air saturates two of alkaline air, and with it forms
the vitriolic ammoniac.

Water imbibes between 30 and 40 times its bulk of this air, and retains
it when frozen. Water thus impregnated dissolves some metals, and
thereby yields inflammable air.

If this water be confined in a glass tube, together with common air, and
be exposed to a long continued heat, it forms real sulphur, the
dephlogisticated part of the common air being imbibed, and forming real
vitriolic acid, which uniting with the phlogiston in the air, forms the
sulphur. Also this air mixed with atmospheric air will, without heat,
imbibe some part of it, and thereby become the common acid of vitriol;
so that water impregnated with vitriolic acid air, commonly called
_sulphureous_, or _phlogisticated acid of vitriol_, wants
dephlogisticated air to make it the common acid of of vitriol.

This kind of air is imbibed by oils, which thereby change their colour;
whale oil becoming red, olive oil of an orange colour, and spirit of
turpentine of the colour of amber.

If this air be confined in a glass tube by mercury, and the electric
spark be taken in it, a black tinge will be given to the glass
contiguous to the spark, and this black substance appears to be mercury
super-phlogisticated; since by exposure to air it becomes running
mercury: so that the vapour of mercury must be diffused through every
part of this air, to the distance of at least several feet from the
surface of the mercury.


_Of Fluor Acid Air._

Fluor acid air is procured by dissolving the earthy substance called
_fluor_ in vitriolic acid.

This kind of air extinguishes a candle, and, like vitriolic acid air,
one measure of it saturates two of alkaline air. It is peculiar to this
kind of air to dissolve glass when it is hot.

It seems to consist of a peculiar acid vapour united to the stony
substance of the fluor; for water being admitted to it absorbs the acid
vapour, and the stony substance is deposited. By this means it exhibits
an amusing appearance, whether water be admitted to a glass jar
previously filled with that air, or the bubbles of air be admitted, as
they are formed, to a quantity of water resting on mercury.




LECTURE X.


_Of Alkaline Air._

Alkaline air is produced by means of heat from caustic volatile alkali,
and also from a mixture of sal-ammoniac and slaked lime, in the
proportion of about one-fourth of the former to three-fourths of the
latter. In this case the marine acid in the sal-ammoniac unites with the
calcareous earth, and the volatile alkali (probably with the assistance
of the water) takes the form of air.

This species of air is heavier than inflammable air, but lighter than
any of the acid airs. Like them, however, it dissolves ice, and
deprives alum, and some other saline substances, of the water which they
contain. United with fixed air, it makes the concrete volatile alkali;
with marine acid air, the common sal-ammoniac; and with water, the
caustic volatile alkali.

The electric spark, or a red heat, converts alkaline air into three
times its bulk of inflammable air; and the calces of metals are revived
in alkaline, as well as in inflammable air; but there remains about
one-fourth of its bulk of phlogisticated air. These facts shew that
alkaline air consists chiefly of phlogiston.


_Miscellaneous Observations relating to Air._

The _nitrous_ acid may be exhibited in the form of air, as well as the
vitriolic, the marine, and the fluor acids. But it cannot be confined
even by mercury, which it instantly dissolves. It may, however, in some
measure, be confined in a dry glass vessel, from which it will in a
great measure expel the common air. This nitrous acid air is that red
vapour, which is produced by the rapid solution of bismuth, and some
other metals in the nitrous acid. But the vegetable acid cannot be
exhibited in the form of air. It is only capable of being converted into
vapour, like water: and in the common temperature of our atmosphere,
returns to a state of fluidity.

Different kinds of air which have no affinity to each other, when once
mixed together will not separate, notwithstanding any difference of
specific gravity. Such is the case of a mixture of inflammable and
dephlogisticated air, and even of inflammable and fixed air. Without
this property also, the phlogisticated air, which constitutes the
greatest part of our atmosphere, being specifically lighter than
dephlogisticated air, of which the other part of it consists, would
separate from it, and ascend into the higher regions of the atmosphere.
Inflammable air, however, will not mix with acid or alkaline air.

Different kinds of air are expanded differently by the same degrees of
heat; dephlogisticated air the least, and alkaline air the most.

If any fluid, as water, spirit of wine, or even mercury, be heated in a
porous earthen vessel, surrounded by any kind of air, the vapour of the
fluid will pass through the vessel _one_ way, while the air passes the
_other_; and when the operation ceases, with respect to the _one_, it
likewise ceases with respect to the _other_.




LECTURE XI.


_Of Liquid Substances_;

AND FIRST OF

_WATER_.

Having considered all the substances that are usually found in the form
of _air_, I come to those that are generally in a _fluid_ form,
beginning with _water_, which is the principal, if not the only cause of
fluidity to all the other substances that I shall place in this class.

Pure water is a liquid substance, transparent, without colour, taste, or
smell; and with different degrees of heat and cold may be made to assume
the three forms of a solid, of a fluid, and of air. Below 32 deg. of
Fahrenheit it is ice, and above 212 deg. it is vapour; so that in an
atmosphere below 32 deg. it never could have been known to be any thing else
than a peculiar kind of stone, and above 212 deg. a peculiar species of
air.

In passing from the state of a solid to that of a liquid, water absorbs
a great quantity of the principle, or matter, of _heat_, which remains
in it in a _latent_ state; and in passing from a state of fluid to that
of vapour, it absorbs much more; and this heat is found when the
processes are reversed. It has been observed, that when water becomes
vapour, it takes the form of small globules, hollow within, so as to be
specifically lighter than air.

The degree of heat at which water is converted into vapour depends upon
the pressure of the atmosphere; so that in vacuo, or on the top of a
high mountain, it boils with little heat; and when compressed, as in
Papin's digester, or in the bottom of a deep pit, it requires much heat.
In the former case the restoring of the pressure will instantly put a
stop to the boiling, and in the latter case the removing of the pressure
will instantly convert the heated water into vapour.

The ease with which water is converted into vapour by heat, has given a
great power to mechanicians, either by employing the natural pressure of
the atmosphere, when steam is condensed under a moveable pistern, in an
iron cylinder, which was the principle of the old fire-engine, or by
employing the elastic power of steam to produce the same effect, which
is the principle of Mr. Watt's steam engine.

Water was long thought to be incompressible by any external force, but
Mr. Canton has shewn that even the pressure of the atmosphere will
condense it very sensibly.

We do not know any external force equal to that by which water is
expanded when it is converted into ice, or into vapour. For though the
particles of water approach nearer by cold, yet when it crystallizes,
the particles arrange themselves in a particular manner, with
interstices between them; so that, on the whole, it takes up more room
than before.

Water has an affinity to, and combines with, almost all natural
substances, aerial, fluid, or solid; but most intimately with acids,
alkalies, calcareous earth, and that calx of iron which is called
_finery cinder_, from which the strongest heat will not expel it.

It has been supposed by some, that by frequent distillation, and also by
agitation, water may be converted into a kind of earth; but this does
not appear to be the case. It has also of late been thought, that water
is resolvable into dephlogisticated and inflammable air; but the
experiments which have been alleged to prove this do not satisfy me; so
that, for any thing that appeared till very lately, water might be
considered as a simple element. By means of heat, however, it seems to
be resolvable into such air as that of which the atmosphere consists,
viz. dephlogisticated and phlogisticated, only with a greater proportion
of the former.

Water, with respect to specific gravity and temperature, has generally
been made the standard to all other substances; its freezing and boiling
points being the limits by means of which thermometers are graduated.
Other substances have also been compared with water, as a standard,
with respect to the capacity of receiving heat, and retaining it in a
latent state, as will be shewn when we consider the subject of heat.




LECTURE XII.


_Of the Nitrous Acid._

Under the head of _liquids_ I shall consider _acids_ and _alkalis_,
though some of them may be exhibited in the form of air, and others in a
solid form. These two chemical principles are formed to unite with one
another, and then they constitute what is called a _neutral salt_.

Both acids and alkalis are distinguishable by their taste. Another test,
and more accurate, is, that acids change the blue juices of vegetables
red, and alkalis turn the syrup of violets green.

Acids are generally distinguished according to the three kingdoms to
which they belong, viz. _mineral_, _vegetable_, and _animal_. The
mineral acids are three, the _nitrous_, the _vitriolic_, and the
_marine_.

The nitrous acid is formed by the union of the purest inflammable air,
or the purest nitrous air, with dephlogisticated air. But it is usually
procured from nitre by means of the vitriolic acid, which, seizing its
base, expels the nitrous acid in a liquid form. On this account this
acid is said to be weaker than the vitriolic.

If the nitrous acid be made to pass through a red-hot earthen tube, it
will be decomposed, and the greatest part of it be converted into
dephlogisticated air.

Like all other acids, the nitrous acid has a strong affinity to water;
but it is not capable of so much concentration as the vitriolic. It is
generally of an orange or yellow colour; but heat will expel this colour
in the form of a red vapour, which is the same acid in the form of air,
and loaded with phlogiston; and therefore when it is colourless it is
said to be dephlogisticated. But the colourless vapour exposed to heat,
or to light, will become  again; and the liquid acid imbibing
this  vapour, becomes  as before. This acid tinges the
skin of a yellow colour, which does not disappear till the epidermis be
changed.

The nitrous acid unites with phlogiston, alkalis, metallic substances,
and calcareous earth.

By means of its affinity with phlogiston it occasions that rapid
accension called _detonation_, when any salt containing this acid,
especially nitre, is applied to hot charcoal, or when charcoal is put to
hot nitre. In fact, the charcoal burns so rapidly by means of the
dephlogisticated air supplied by the nitre.

A mixture of sulphur assists the accension of these substances, and
makes gunpowder, in the explosion of which much nitrous or
phlogisticated air is suddenly produced, and expanded by the heat. The
application of this force, both to useful and destructive purposes, is
well known. If, instead of nitre, a salt made with dephlogisticated
marine acid be made use of, the explosion is more easily produced, and
is much more violent. Friction will do this as well as heat.

Nitre also enters into the composition of _pulvis fulminans_, viz. three
parts nitre, two of dry alkali, and one of sulphur. This composition
melts, and yields a blue flame, before it explodes.

By means of the affinity of the nitrous acid to _oil_, another substance
containing phlogiston, it is capable of producing not only a great heat,
but even a sudden flame, especially when mixed with a little vitriolic
acid.

Nitrous acid dissolves all metallic substances except gold and platina,
and in the solution nitrous air is produced.

The particular kinds of saline substances formed by the union of the
nitrous acid with the several metals and earths may be seen in tables
constructed for the purpose. They are all deliquescent.




LECTURE XIII.


_Of the Vitriolic Acid._

The vitriolic acid, so called because it was originally procured from
_vitriol_, is now generally procured from sulphur; the dephlogisticated
part of the air uniting with it in the act of burning.

That dephlogisticated air is essential to this acid is evident from the
decomposition of it; for if the vapour of it be made to pass through a
red-hot earthen tube, a great quantity of dephlogisticated air is
procured.

This acid has a strong affinity to water, with which it unites with much
heat; and it is capable of greater concentration, or of being made
specifically heavier, than any other acid. When pure, it is entirely
free from colour and smell, owing, probably, to its being free from
phlogiston, which is inseparable from the nitrous or marine acids.

The vitriolic acid will dislodge the nitrous, or marine, or any other
acid, from their earthy or metallic bases; from which property it is
called the strongest of all the acids.

By means of the superior affinity of the vitriolic acid to earths, and
especially to _terra ponderosa_, the smallest quantity of it in water
may be discovered by a solution of this earth in the marine acid. In
this acid the terra ponderosa is held in perfect solution; but with the
vitriolic acid it forms a substance that is insoluble in water, and
therefore it instantly appears in the form of a white cloud.

Perhaps chiefly from the strong affinity which this acid has with water,
_pyrophorus_, consisting of a mixture of alum and several substances
containing phlogiston, takes fire spontaneously on exposure to the air.
It is commonly made of three parts of alum and one of brown sugar, or of
two parts alum, one of salt of tartar, and one of charcoal. They must be
heated till they have for some time emitted a vapour that burns with a
blue flame.

The saline substances produced by the union of this acid with the
several earths and metals, are best exhibited in tables constructed for
the purpose. When united to three of the metals, viz. iron, copper, and
zinc, they are called _vitriols_, green, blue, and white. And all the
substances which this acid unites with crystallize, and do not
deliquesce.

This acid unites with oil, and the mixture is always black.

When any substance containing phlogiston is heated in the vitriolic
acid, another species of the acid, called _sulphureous_, is formed, of a
pungent smell. In reality, it is water impregnated with vitriolic acid
air. It makes, however, a distinct species of acid, and is dislodged
from its base by most of the others.


_Of the Marine Acid._

The marine acid is procured from common salt by the vitriolic acid,
which unites with its base, the fossil alkali.

This acid is generally of a straw-colour; but this is owing to an
impregnation with some earthy matter, most of which it readily
dissolves, especially the metallic ones. It is less capable of
concentration than the vitriolic or nitrous acids, perhaps from a more
intimate union of phlogiston with it. No heat can extract from it any
dephlogisticated air.

Though this is denominated a weaker acid than the nitrous, yet it will
take silver, lead, or mercury, from their union with the nitrous acid.
Upon this principle, a solution of these metals in the nitrous acid will
readily discover whether any water contains the marine acid, the latter
uniting with the metal dissolved in the former, and forming with it, if
it be silver, a _luna cornea_; which being a substance insoluble in
water, discovers itself by a cloudy appearance.

The union of the marine acid with earths forms salts that easily
deliquesce, but with the metals such as are capable of crystallization;
and so also is that formed by the union of this acid to terra ponderosa.

Neither this acid nor the nitrous will dissolve gold or platina; but a
mixture of them, called _aqua regia_, will do it.

The marine acid has a strong affinity to dephlogisticated air, and will
take it from manganese and other substances; and with this union it
becomes a different acid, called _dephlogisticated marine acid_, being
water impregnated with dephlogisticated marine acid air, described
above.




LECTURE XIV.


_Of the Vegetable Acids, and others of a less perfect nature._

The principal of the vegetable acids are the _acetous_ and the
_tartareous_. The acetous acid is the produce of a peculiar fermentation
of vegetable substances, succeeding the _vinous_, in which ardent spirit
it is procured, and succeeded by the _putrefactive_, in which volatile
alkali is generated.

Thus wine is converted into vinegar. Crude vinegar, however, contains
some ingredient from the vegetable substances from which it was
procured: but distillation separates them, and makes the vinegar
colourless; though some of the acid is lost in the process.

The acetous acid is concentrated by frost, which does not affect the
proper acid, but only the water with which it is united. It may likewise
be concentrated by being first combined with alkalies, earths, or
metals, and then dislodged by a stronger acid, or by mere heat. Thus the
acetous acid, combined with vegetable alkali, forms a substance that is
called the _foliated earth of tartar_; and it may be expelled from it by
the vitriolic acid. When combined with copper it makes _verdigris_; and
from this union heat alone will expel it in a concentrated state. The
acetous acid thus concentrated is called _radical vinegar_. Still,
however, it is weaker than any of the preceding mineral acids.

Several vegetables, as lemons, sorrel, and unripe fruit, contain acids,
ready formed by nature, mixed with some of the essential oil of the
plants, which gives them their peculiar flavours. All these acids have
peculiar properties; but it is not necessary to note them in this very
general view of the subject. Like vinegar, these acids may be
concentrated by frost, and also by a combination with other substances,
and then expelled by a stronger acid.

The _acid of tartar_ is very similar to that of vinegar. Tartar, from
which it is procured, is a substance deposited on the inside of
wine-casks, though it is also found ready formed in several vegetables.
It consists of the vegetable alkali and this peculiar acid. When refined
from its impurities, it is called _crystals_, or _cream of tartar_. The
acid is procured by mixing the tartar with chalk, or lime, which imbibes
the superfluous acid, and this is expelled by the acid of vitriol. Or it
may be procured by boiling the tartar with five or six times its weight
of water, and then putting the acid of vitriol to it. This unites with
the vegetable alkali, and forms vitriolated tartar; and the pure acid of
tartar may be procured in crystals, by evaporation and filtration, equal
in weight to half the cream of tartar. This acid of tartar is more
soluble in water than the cream of tartar.

This acid, united to the mineral alkali, makes _Rochelle salt_.

Every kind of wood, when distilled, or burned, yields a peculiar acid;
and it is the vapour of this acid that is so offensive to the eyes in
the smoke of wood.

A peculiar acid is obtained from most vegetable substances, especially
the farinaceous ones, and from sugar, by distillation with the nitrous
acid. This seizes upon the substance with which the acid was united, and
especially the phlogiston adhering to it, and then the peculiar _acid of
sugar_ crystallizes. Thus with three parts of sugar, and thirty of
nitrous acid, one part of the proper acid of sugar may be obtained. By
the same process an acid may be procured from camphor.

The _bark of oak_, and some other vegetable substances, especially
nut-galls, contain a substance which has obtained the name of _the
astringent principle_; the peculiar property of which is, that it
precipitates solutions of iron in the form of a black powder, and in
this manner _ink_ is made. But by solution in water and evaporation,
crystals, which are a proper _acid of galls_, may be obtained.

_Amber_ is a hard semitransparent substance, chiefly found in Prussia,
either dug out of the earth, or thrown up by the sea. It is chiefly
remarkable for its electrical property; but by distillation in close
vessels there sublimes from it a concreted acid, soluble in 24 times
its weight of cold water. Amber seems to be of vegetable origin, and to
consist of an oil united to this peculiar acid.

The acids I shall mention next are of a mineral origin; but being of a
less perfect nature as acids, I shall only just note them here.

_Borax_ is a substance chiefly found in a crystallized state in some
lakes in the East Indies. It consists of the mineral alkali and a
peculiar acid, which may be separated, and exhibited in white flakes, by
putting acid of vitriol to a solution of it in water. This acid has been
called _sedative salt_, from its supposed uses in medicine. It is an
acid that requires fifty times its weight of water to dissolve it.

Several other mineral substances, as _arsenic_, _molybdena_, _tungsten_,
and _wolfram_, in consequence of being treated as the preceding
vegetables ones, have been lately found to yield peculiar acids. They
are also produced in a concrete state, and require a considerable
proportion of water to make them liquid; but as the water in which they
are dissolved turns the juice of litmus red, and as they also unite
with alkalis, they have all the necessary characteristics of acids.




LECTURE XV.


_Of the Phosphoric Acid._

The most important acid of _animal_ origin, though it has lately been
found in some mineral substances, is the _phosphoric_.

Phosphorus itself is a remarkable substance, much resembling sulphur,
but much more inflammable. It has been procured chiefly, till of late,
from urine, but now more generally from _bones_, by means of the
vitriolic acid, which unites with the calcareous earth of which bones
consist, and sets at liberty the phosphoric acid, or the base of that
acid, with which it was naturally combined. The acid thus procured,
mixed with charcoal, and exposed to a strong heat, makes phosphorus.

This substance burns with a lambent flame in the common temperature of
our atmosphere, but with a strong and vivid flame if it be exposed to
the open air when moderately warm. In burning it unites with the
dephlogisticated air of the atmosphere, and in this manner the purest
phosphoric acid is produced.

This acid is also procured in great purity by means of the nitrous or
vitriolic acids, especially the former, which readily combines with the
phlogiston of the phosphorus, and thus leaves the acid pure. In this
process phlogisticated air is produced.

This acid is perfectly colourless, and when exposed to heat loses all
its water, and becomes a glassy substance, not liable to be dissipated
by fire, and readily uniting with earths.

United to the mineral alkali, it forms a neutral salt, lately introduced
into medicine. United to the mineral and vegetable alkalis naturally
contained in urine, it has obtained the name of _microcosmic salt_,
frequently used as a flux for mineral substances with a blow-pipe.

Besides the phosphoric, there are other acids of an animal origin; as
that of _milk_, that of _sugar of milk_, that of the _animal calculus_,
and that of _fat_.

The acid of milk is the sour whey contained in butter-milk, which, by a
tedious chemical process, may be obtained pure from any foreign
substance.

The sugar of milk is procured by evaporating the whey to dryness, then
dissolving it in water, clarifying it with whites of eggs, and
evaporating it to the consistence of honey. In this state white crystals
of the acid of sugar of milk will be obtained.

By distilling these crystals with nitrous acid, other crystals of the
proper _acid of sugar of milk_ will be obtained, similar to those of the
acid of sugar.

If the human calculus be distilled, it yields a volatile alkali, and
something sublimes from it which has a sourish taste, and therefore
called the _acid of the calculus_. It is probably some modification of
the phosphoric acid.

Animal fat yields an acid by distillation, or by first combining it with
quick-lime, and then separating it by the vitriolic acid. Siliceous
earth is corroded by this acid.




LECTURE XVI.


_Of Alkalis._

The class of substances that seems particularly formed by nature to
unite with acids, and thereby form _neutral salts_, are the _alkalis_.
They have all a peculiar acrid taste, not easily defined. They change
the blue juices of vegetables green, or purple, and in common with acids
have an affinity with water, so as to be capable of being exhibited in a
liquid form; though when this water is expelled by heat, some of them
will assume a solid form.

Alkalis are of two kinds; the _fixed_, which have no smell, and the
_volatile_ which have a pungent one.

The fixed alkalis are of _vegetable_ or _mineral_ origin. When in a
solid form, they both melt with a moderate heat, and uniting with earthy
substances, make _glass_. With an intense heat they are volatilized.

Vegetable alkali is procured by burning plants, and lixiviating the
ashes; a purer kind by the burning of tartar, hence called _salt of
tartar_; but the purest of all is got by the deflagration of nitre; the
charcoal uniting with the acid as it assumes the form of
dephlogisticated air, and the alkali being left behind.

Mineral alkali is found in ashes of sea-weed. It is likewise the basis
of sea-salt; from which it is separated by several processes, but
especially by the calx of lead, which has a stronger affinity with the
marine acid with which it is found combined.

Alkalis united with fixed air are said to be _mild_, and when deprived
of it _caustic_, from their readiness to unite with, and thereby
_corrode_, vegetable and animal substances. To render them caustic, they
are deprived of their fixed air by quick-lime; and in this state they
unite with oils, and make _soap_.

Alkalis have a stronger affinity with acids than metals have with them;
so that they will precipitate them from their solutions in acid
menstruums.

The vegetable fixed alkali has a strong attraction to water, with which
it will become saturated in the common state of our atmosphere, when it
is said to _deliquesce_; and having the appearance of _oil_, the salt of
tartar is thus said to become _oil of tartar per deliquium_. On the
other hand, the mineral, or fossil alkali, is apt to lose its water in a
dry atmosphere, and then it is said to _effloresce_. In this state it is
often found on old walls.

Volatile alkali is procured by burning animal substances; in Egypt (from
whence, as contained in _sal ammoniac_, we till of late imported it)
from camel's dung; but now from bones, by distillation. To the liquor
thus procured they add vitriolic acid, or substances which contain it.
This acid unites with the alkali, and common salt being put to it, a
double affinity takes place. The vitriolic acid uniting with the mineral
alkali of the salt, makes _Glauber salt_, and the marine acid uniting
with the volatile alkali, makes _sal ammoniac_. Slaked lime added to
this, unites with the marine acid of the ammoniac, and sets loose the
volatile alkali in the form of _alkaline air_, which combining with
water, makes the liquid caustic volatile alkali. If chalk (containing
calcareous earth united with fixed air) be mixed with the sal ammoniac,
heat will make the calcareous earth unite with the marine acid, while
the fixed air of the chalk will unite with the volatile alkali, and
assume a solid form, being the _sal volatile_ of the apothecaries.




LECTURE XVII.


_Of Liquid Inflammable Substances._

Of liquid inflammable substances the principal is _spirit of wine_,
sometimes called _ardent spirit_, and, when highly rectified, _alcohol_.
It is obtained from vegetable substances by their going through the
vinous fermentation. It is considerably lighter than water, colourless,
and transparent, has a peculiar smell and taste, and the property of
inebriating.

Ardent spirit seems to consist of a peculiar combination of phlogiston
and water; for when the vapour of it is made to pass through a red-hot
earthen tube, it is resolved into water and inflammable air. It is
highly inflammable, and burns without smoke, or leaving any residuum;
and in the act of burning its phlogiston so unites with dephlogisticated
air as to make fixed air.

Ardent spirit mixes readily with water in all proportions, and also with
essential oils, and balsams or resins, which are the same thing
inspissated.

By its affinity with essential oils, ardent spirit extracts them froth
aromatic plants; and these liquors have obtained the name of
_tinctures_.

When the tinctures are distilled, the more volatile parts of the
essential oils, which come over in distillation, have acquired the name
of _waters_; as _Lavender water_, _Rosemary water_, &c. and what remains
in the still is called the _extract_ of the plant. If the tinctures be
diluted with much water, the resinous part of the plant will be obtained
pure, and separated from the extractive part, which will remain
dissolved in the water, while the resin separates from it.

Spirit of wine will not dissolve the gummy parts of vegetables; and by
this means the gummy substances may be separated from their solutions in
water, the spirit uniting with the water only. On the other hand, if
resins be dissolved in spirit of wine, the affusion of water will
separate them. By means of the affinity of spirit of wine with water, it
will seize upon the water in which several salts are dissolved, and thus
produce an instant crystallization of them.

Salt of tartar has a greater affinity to water than spirit of wine, and
by extracting water from it, will assist in concentrating it; but the
best method of rendering spirit wine free from water is distillation,
the ardent spirit rising before the water.

Spirit of wine mixed with the vitriolic and other mineral acids, renders
them milder, and thereby more proper for certain medicinal uses. This is
called _dulcifying_ them.

Spirit of wine is a powerful antiseptic, and is therefore of use to
preserve vegetable and animal substances from putrefaction.


_Of AEther._

If spirit of wine be distilled with almost any of the acids, the produce
is a liquor which has obtained the name of _AEther_, from its extreme
lightness and volatility, being much lighter, and more volatile, than
any other fluid that we are acquainted with. It is highly inflammable,
but the burning of it is accompanied with smoke, and some soot; and on
this account it is a medium between spirit of wine and oil, the acid
having taken from the spirit of wine part of the water that was
essential to it, at the same time that it communicated something of its
acid peculiarly modified; since aethers have different properties
according to the acids by which they are made; as the _vitriolic_, the
_nitrous_, the _marine_, and the _acetous_. No aether, however, can be
made from the marine acid till it has been in some measure
dephlogisticated; from which it may be inferred, that dephlogisticated
air is necessary to the composition of aether. Vitriolic aether is the
most common, in consequence of the process by which it is made being the
easiest.

AEther does not mix with water in all proportions, like spirit of wine,
but ten parts of water will take up one of aether. It easily mixes with
all oils.

It is something remarkable, that though aether will not dissolve gold, it
will take from aqua regia the gold that has been previously dissolved in
it.

By the quick evaporation of aether a considerable degree of cold may be
procured; and on this principle it has sometimes been applied to relieve
the head-ach and other pains.




LECTURE XVIII.


_Of Oil._

Oil is a liquid inflammable substance, of great tenacity, disposed to
pour in a stream rather than in drops. It is little, if at all, soluble
in water. It burns with smoke and soot, and leaves a residuum of a coaly
substance. It consists of acid and water combined with phlogiston.

All oil is the produce of the vegetable or animal kingdom, no proper
mineral substance containing any of it.

By distillation oil is in part decomposed, and by this means the thicker
kinds of oil are rendered thinner and more volatile, the acid, to which
their consistence is chiefly owing, being lost in the process. By
repeated distillation it is supposed that all oils may be brought almost
to the state of aether, and even of ardent spirit.

Acids act powerfully upon oils, but very differently, according to the
nature of each. Alkalies also combine with oils, and the less thin and
volatile they are, the more easily are they soluble in alkalies. The
union of alkali and oil makes _soap_. All oil dissolves sulphur, and
with it makes what is called a _balsam_. Oils also dissolve metallic
substances, but most sensibly copper and lead. United with the calx of
lead, it is used in painting.

Oil not readily mixing with water, it will diffuse itself over its
surface, and, notwithstanding its tenacity, it will do this very
rapidly, and to a great extent; and then it has the extraordinary
effect of preventing the action of the wind upon the water, so as to
prevent the forming of waves. If a quantity of oil and water be put into
a glass vessel and swung, the surface of the water below the oil will be
seen to change with respect to the vessel, but not that of the oil. If
spirit of wine be put upon them, that will be at rest, and both the
lower fluids in motion.

Vegetable oil is of two kinds, the _soft_, or _mild_, which has little
or no taste or smell, and the _essential_ oil, which is thin, and
retains the smell and taste of the plant from which it was extracted.

Mild or sweet oil is expressed from the grains or kernels of vegetables,
and requires a considerable degree of heat to convert it into vapour, in
which state alone it is capable of being inflamed.

_Essential oil_ is volatile in the heat of boiling water, and is
generally obtained by means of distillation from the most odoriferous
sorts of plants; but is sometimes found in their vesicles, as in the
rind of an orange. The strong taste of this kind of oil arises from the
disengaged acid which abounds in it; and by this means it is soluble in
spirit of wine, which sweet oil is not; but it loses much of this
property by repeated distillations. By long exposure to the air it loses
its more volatile parts, and thereby approaches to the nature of a
resin. This volatile odoriferous principle has been called the _spiritus
rector_ of the plant.

The essential oils of different plants differ much in their specific
gravity, and also in the manner by which they are affected by cold, some
being heavier and others lighter than water, and some being more
difficultly, and others more easily, congealed. Though the differences
with respect to _weight_ and _consistency_ in these oils is probably
owing to the state of the acid that is combined with them, these two
properties are wholly independent of each other; some essential oils
being very thin and yet heavy, and others thick and yet light. Essential
oils are used in perfumes, and also in medicine, acting powerfully the
nervous system.

Essential oils are very apt to be adulterated. If it be with sweet oil,
it may be discovered by evaporation on white paper, or by a solution in
spirit of wine, which will not act upon the sweet oil. If spirit of wine
be mixed with it, it will be discovered by a milky appearance upon
putting water to it, which uniting with the spirit, will leave the oil
much divided. If oil of turpentine, which is the cheapest of essential
oils, be mixed with any of the more valuable kinds, it will be
discovered by evaporation; a strong smell of turpentine being left on
the paper, or cloth, upon which the evaporation was made.

Animal oil, like the vegetable, is of two kinds; the first _butter_, or
_fat_, which is easily congealed, owing to the quantity of acid that is
intimately combined with it. It resembles the sweet oil of vegetables in
having no smell or taste. The other kind of animal oil is extracted by
distillation from the flesh, the tendons, the bones, and horns, &c. of
animals. It differs essentially from the other kind of animal oil, by
containing an alkali instead of an acid. By repeated distillation it
becomes highly attenuated and volatile; and in this state it is called
the _oil of Dippel_, the discoverer of it.

All oil exposed to much heat is in part decomposed, and acquires a
disagreeable smell; and in this state it is said to be _empyreumatic_:
but this property is lost by repeated distillations.

Besides the vegetable and animal oils above described, there is a fossil
oil called _bitumen_, the several kinds of which differ much in colour
and consistence; the most liquid is called _petroleum_, from being found
in the cavities of rocks, and the more solid kinds are _amber_, _jet_,
_asphaltum_, and _pit-coal_. When distilled, the principal component
parts of all these substances are an oil and an acid. But all fossil oil
is probably of vegetable or animal origin, from masses of vegetables or
animals long buried in the earth. Their differences from resins and
other oily matters are probably owing to _time_; the combinations of
mineral acids and oils so nearly resembling bitumens, the principal
difference being their insolubility in spirit of wine.

That the most solid of these, as amber, has been formerly in a liquid
state, is evident, from insects and other substances being frequently
found in them; and pit-coal has been often found with both the internal
texture and external appearance of wood; so that strata of pit-coal have
probably been beds of peat in some former state of the earth.




LECTURE XIX.


_Of Solid Substances._

All solid substances are capable of becoming fluid by heat, and most of
them may thereby be reduced into a state of vapour, or air; and in
passing from a fluid into a solid state their component parts assume a
particular mode of arrangement, called _crystallization_, which differs
according to the nature of the substance; so that all solids, especially
if they be suffered to concrete slowly, may be called _crystals_.

Exclusive of _salts_, which have been considered already, as formed by
the union of acids and alkalis, solids in general have obtained the
names of _earths_, or _stones_, which differ only in their texture; and
they are distinguished into those that are _metallizable_, or those that
are not; the former being called _ores_, and the latter simply _earths_;
the principal of which are the _calcareous_, _siliceous_,
_argillaceous_, _magnesia_, _terra ponderosa_, and a few others which
have been discovered lately, but have not been much examined.


_Of Calcareous Earth._

Calcareous earth is found in the shells of fishes, the bones of animals,
chalk, lime-stone, marble, and gypsum: but all calcareous earth is
supposed to be of animal origin; and beds of chalk, lime-stone, or
marble, are thought to have been beds of shells formed in the sea, in
some pristine state of the earth.

The calcareous earth which is found in shells, lime-stone, and marble,
is combined with fixed air, discovered by effervescing with acids. To
obtain it perfectly pure, the earth must be pounded and washed with
water, in order to free it from any saline substance which may be
contained in it, then dissolved in distilled vinegar, and precipitated
by mild alkalies. Lime-stone exposed to heat loses about half its
weight, in fixed air and water, and the remainder, called _quick-lime_,
attracts water very powerfully, and their union is attended with much
heat, after which it dissolves into a fine powder called _slaked lime_.
If it be left exposed to the atmosphere, it will of itself, by gradually
imbibing moisture, fall into the state of powder.

Water dissolves about one seven hundredth part of its weight of
quick-lime, and is then called _lime-water_. Exposed to the air, a crust
will be formed on its surface, which is found to consist of calcareous
earth and fixed air.

Lime and water mixed with sand make _mortar_, by which means different
stones may be made to cohere as one mass, which is the most valuable
use of this kind of earth.

Calcareous earth, united with vitriolic acid, makes _gypsum_; and this
substance pounded and exposed to heat, parts with its water, and is then
called _plaister of Paris_. In this state, by imbibing water again, it
becomes a firm substance, and thus is useful in making moulds, &c.

The earth of animal bones is calcareous united to the phosphoric acid.


_Of Siliceous Earth._

Siliceous earth seems to be formed by nature from chalk, perhaps by the
introduction of some unknown acid, which the vitriolic acid is not able
to dislodge. It abounds in most substances which are hard enough to
strike fire with steel, as _flint_, _rock crystal_, and most _precious
stones_. It is not acted upon by any acid except the fluor and
phosphoric, but especially the former: but it is soluble in alkalies;
and being then dissolved in water, makes _liquor silicum_, from which
the purest siliceous earth may be precipitated by acids. For this
purpose about four times the weight of alkali must be made use of. With
about equal weights of alkali and siliceous sand is made _glass_, of so
great use in admitting light and excluding the weather from our houses,
as well as for making various useful utensils. To make glass perfectly
colourless, and at the same time more dense, commonly called _flint
glass_, manufacturers use a certain proportion of calx of lead and
manganese.

Siliceous earth is not affected by the strongest heat, except by means
of a burning lens, or dephlogisticated air.




LECTURE XX.


_Of Argillaceous Earth._

Argillaceous earth is found in _clay_, _schistus_, or _slate_, and in
_mica_; but the purest is that which is precipitated from a solution of
alum by alkalies; for alum consists of the union of vitriolic acid and
argillaceous earth.

This species of earth is ductile with water; it then hardens and
contracts by heat, so as to be of the greatest use in forming _bricks_,
or stones of any required form or size. By means of the property of clay
to contract in the fire, Mr. Wedgwood has constructed an excellent
thermometer to measure the degrees of extreme heat.

The ductility of clay seems to depend upon some acid, probably the
vitriolic, adhering to it; for it loses that property when it is burned
into a brick, but recovers it when it has been again dissolved in an
acid.


_Of Terra Ponderosa._

_Terra ponderosa_, or _marmor metallicum_, is generally found in two
states, viz. united to vitriolic acid, when it is called _calk_, or to
fixed air, when it is called _terra ponderosa aerata_.

To obtain it pure from its union with the vitriolic acid, it must be
melted with about twice its weight of fixed alkali; which unites with
the acid, and forming a saline substance, may be washed out of it. In
this state it contains water, and therefore, when exposed to heat, will
yield fixed air; whereas the terra ponderosa aerata will not yield fixed
air by heat only, but when steam is made to pass over it when red hot.
This proves that water is essential to the composition of fixed air.

This stone is distinguishable by its great specific gravity, being four
times as heavy as water; but though in this it resembles an _ore_, it
has not been found to be metallizable.


_Of Magnesia._

This species of earth is found in _steatites_, or _soap rock_, _Spanish
chalk_, _asbestus_, and _Muscovy talck_; but the purest is got by
dissolving _Epsom salts_ (which consists of this earth united to the
vitriolic acid) and precipitating it by a mild alkali. In this state it
becomes united to fixed air, which may be expelled by heat. It is then
_calcined_, or _caustic_, but differs from quick-lime by not being
soluble in water.

_Asbestus_, which contains much of this kind of earth, is remarkable for
not being destructible by heat, though it is sometimes found in flexible
fibres, so as to be capable of being woven into cloth.

_Muscovy talck_ is remarkable for the thin and transparent flakes into
which it is divisible, and thereby capable of various uses.

There are some other distinct species of earth, particularly one brought
from Botany Bay, and another called _Stontiate_, from the place where it
was found in Scotland; but they have not as yet been much examined.

All stones formed by nature are compounded, and to distinguish them from
one another, and ascertain the parts of which they consist, is the
subject of _lithology_, a very extensive branch of knowledge.

All the simple earths are nearly, if not absolutely, _infusible_; but
when they are mixed they may all be fused.




LECTURE XXI.


_Of Ores_.

Metallizable earths, commonly called _ores_, when united to phlogiston,
make the metals, distinguishable for their specific gravity, their
opacity, shining appearance, and fusibility.

All the proper metals are _malleable_, and those which are not so are
called _semi-metals_.

The metals again are subdivided into the _perfect_ and _imperfect_. The
former, which are _gold_, _silver_, and _platina_, suffer no change by
fusion, or the longest continued heat: whereas heat calcines or
dissipates the phlogiston of the imperfect metals, which are _mercury_,
_lead_, _copper_, _iron_, and _tin_, so that they return to the state of
earth; and this earth is always heavier than the metal, though of less
specific gravity, having received an addition of weight from water or
air: but these earths, or ores, being exposed to heat in contact with
substances containing phlogiston, again become metals, and are then said
to be _revived_.

The semi-metals are _bismuth_, _zinc_, _nickel_, regulus of _arsenic_,
of _cobalt_, of _antimony_, of _manganese_, of _wolfram_, and of
_molybdena_.

All metallic substances are crystallizable, and each in a peculiar form,
which is discovered by leaving a hole in the bottom of the crucible in
which they are melted, and drawing out the stopper, when the mass is
beginning to lose its fluidity.

Some of the metals will not unite to others when hot, and others of them
will; and such as will unite with others are called _solders_. Thus tin
is a solder for lead, and brass, gold, or silver, for iron.

Ores are never found in regular strata, like the different kinds of
earth; but in places which have formerly been cavities, running in all
directions, with respect to the regular strata, and commonly called
_veins_.

Many of the ores in their natural state are said to be _mineralized_
with arsenic or sulphur, those substances being intimately united with
the metallic earths.

In order to convert the ores into metals, some of them are first reduced
to powder, to wash out the earthy or saline particles. They are then
kept in a red heat, which the workmen call _roasting_, in order to drive
away the arsenic, or sulphur, which are volatile; and in the last place
they are fused in contact with charcoal, or other substances containing
phlogiston; and to promote the fusion, lime-stone is frequently mixed
with them. When the operation is completed, the unmetallic parts are
converted into glass, or _scoria_, which lies on the surface, whereas
the metal is found at the bottom.

To discover the quantity of metal in a small piece of ore is called
_assaying_.

When metals are fused together, the specific gravity, fusibility, and
other properties are changed, and in such a manner as could not be
discovered from the properties of the constituent parts.


_Of Gold._

Gold is the heaviest of all metallic bodies except platina. It appears
yellow or reddish by reflected light, but green or blue by transmitted
light, when it is reduced to thin plates.

Though gold undergoes no change in a common furnace, or burning lens, it
may, in part, at least, be calcined by the electric shock.

Gold has the greatest _ductility_, and in wires of equal diameters, it
has the greatest _tenacity_, of all the metals. One grain of it may be
made to cover 56 square inches; some gold leaf being less than a
200,000th part of an inch thick; and when it is made to cover a silver
wire, the gold upon it may not be more than one twelfth part of the
thickness of the gold leaf.

This metal is soluble in aqua regia; and being precipitated by a
volatile alkali, makes a powder called _aurum fulminans_, which is one
fourth heavier than the gold, and explodes with great violence in a heat
something greater than that of boiling water.

Tin precipitates gold in the form of a purple powder, called the _powder
of Cassius_, from the inventor of it, and is used in enamels, or the
glassy coating which is given to metals by heat.

Gold unites with most of the metals, especially with mercury, and these
mixtures are called _amalgams_. In gilding, the amalgam is applied to
the surface of the metal to be gilded, and the mercury is driven off by
heat, leaving the gold attached to the surface.

Gold mixed with iron, makes it harder, for the purpose of cutting
instruments.

To separate gold from the imperfect metals, such as copper, &c. it is
mixed with lead, and then exposed to a strong heat, which calcines the
lead, and with it the imperfect metals, leaving the gold pure. This
process is called _cupellation_, from being performed in a small
crucible called a _cupell_. When the gold is mixed with silver, three
parts more of silver are put to it, and then the silver is dissolved by
nitrous acid, leaving the gold pure. This process is called
_quartation_, from the gold being one fourth part of the mass.

The fineness of gold is generally estimated by dividing the gold into
twenty-four parts, called _carats_. The phrase twenty-three carats fine
means that the mass contains twenty-three parts out of twenty-four of
pure gold, the remainder being _alloy_, of some baser metal. The
fineness of gold may in some measure be discovered by the colour it
leaves upon a _touch-stone_, or fine-grained basaltes.

Gold is generally found nearly pure, but mixed with earth, or diffused
in fine grains through stones.




LECTURE XXII.


_Of Silver._

Silver is the whitest of all the metals, very ductile, but less so than
gold; the thinnest leaves of it being one third thicker than those of
gold. It is not calcined in the heat of a common furnace, but partially
so by repeated fusion, or a strong burning lens.

Sulphureous fumes unite with silver, and tinge it black. The nitrous
acid dissolves it, and will hold more than half its weight of it in
solution. When fully saturated, this solution deposits crystals, which
are called _lunar nitre_, or _nitre of silver_. When these crystals are
melted, and the water they contain driven off, a black substance,
called _lapis infernalis_, or _lunar caustic_, is formed. This is used
as a cautery in surgery. A strong heat will decompose this lunar nitre,
and recover the silver.

Though the nitrous acid dissolves silver the most readily, the marine
acid will deprive the nitrous of it, and form a substance called _luna
cornea_, because, when it is melted and cold, it becomes a transparent
mass something resembling _horn_. From this luna cornea the purest
silver may be obtained. The vitriolic acid will likewise deprive the
nitrous of the silver contained in it, and form a white powder, not
easily soluble in water.

A fulminating silver may be made by the following process: the silver
must first be dissolved in pale nitrous acid, then precipitated by
lime-water, dried, and exposed to the air three days. It must then be
washed in caustic volatile alkali, after which the fluid must be
decanted, and the black powder left to dry in the air. The slightest
friction will cause this powder to fulminate. It is said, that even a
drop of water falling upon it will produce this effect; so that it
ought to be made only in very small quantities, and managed with the
greatest caution.

Most of the metals precipitate silver. That by mercury may be made to
assume the form of a tree, called _arbor Dianae_.

Silver is found native in Peru; and the ores frequently contain sulphur,
or arsenic, or both.


_Of Platina._

Platina is a metal lately discovered in the gold mines of Mexico, where
it is found in small particles, never exceeding the size of a pea, mixed
with ferruginous sand and quartz.

The strongest fire will not melt these grains, though it will make them
cohere; but they may be melted by a burning lens, or a blow-pipe
supplied with dephlogisticated air.

Pure platina is the heaviest body in nature, its specific gravity
exceeding twenty-two. It is very malleable, though considerably harder
than gold or silver, and has the property of welding in common with
iron. This metal is not affected by exposure to the air, or by any
simple acid, though concentrated and hot; but it is dissolved by
dephlogisticated marine acid, and by aqua regia, in which a little
nitrous air is procured. The solution is brown, and when diluted yellow.
This liquor is very corrosive, and tinges animal substances of a
blackish brown colour. Platina is precipitated from a solution in aqua
regia by sal-ammoniac, as gold is by martial vitriol. Iron is
precipitated from this solution by the Prussian alkali. Also most of the
metals precipitate platina, but not in its metallic state.

Arsenic facilitates the solution of platina; and by melting it with
equal parts of arsenic and vegetable alkali, and then reducing the mass
to a powder, it may be made to take any form; and a strong heat will
dissipate the arsenic and the alkali, leaving the platina in the shape
required, not fusible by any heat in a common furnace.

Platina does not readily combine with gold or silver, and it resists the
action of mercury as much as iron; but it mixes well with lead, making
it less ductile, and even brittle, according to the proportion of the
platina. With copper it forms a compound which takes a beautiful polish,
not liable to tarnish, and is therefore used with advantage for mirrors
of reflecting telescopes. It unites easily with tin, and also with
bismuth, antimony, and zinc.




LECTURE XXIII.


_Of Mercury._

Mercury is the most fusible of all the metals, not becoming solid but in
40 deg. below 0 in Fahrenheit's thermometer. It is then malleable. It is
heavier than any other metal except gold or platina. It is volatile in a
temperature much lower than that of boiling water, and in vacuo in the
common temperature of the atmosphere; and at six hundred it boils.

In a degree of heat in which it would rise easily in vapour, mercury
imbibes pure air, and becomes a red calx, called _precipitate per se_.
At a greater degree of heat it parts with that air, and is running
mercury again.

Mercury is not perceptibly altered by exposure to the air.

Mercury is acted upon by the vitriolic acid when hot. In this process
vitriolic acid air is procured, and the mercury is converted into a
white substance, which being dipped in water becomes yellow, called
_turbith mineral_, one third heavier than the mercury from which it was
made. By heat this substance parts with its pure air, and becomes
running mercury; but if the process be made in a clean earthen vessel,
there will remain a portion of _red calx_, which cannot be reduced by
any degree of heat, except in contact with some substance containing
phlogiston. If this be done with a burning lens, in inflammable air,
much of the air will be absorbed.

Mercury is dissolved most readily in the nitrous acid, when the purest
nitrous air is procured; and there remains a substance which is first
yellow, and by continuance red, called _red precipitate_. In a greater
degree of heat the dephlogisticated air will be recovered, and the
mercury be revived; but the substance yields nitrous air after it
becomes solid, and till it changes from yellow to red.

The precipitates of mercury from acids by means of alkalies possess the
property of exploding, when they are exposed to a gradual heat in an
iron spoon, after having been triturated with one sixth of their weight
of the flowers of sulphur. The residuum consists of a violet-
powder, which, by sublimation, is converted into cinnabar.

It seems, therefore, as if the sulphur combined suddenly with the
mercury, and expelled the dephlogisticated air in an elastic state.

The marine acid seizes upon mercury dissolved in nitrous acid, and if
the acid be dephlogisticated, the precipitate is _corrosive sublimate_;
but with common marine acid, it is called _calomel_, or _mercurius
dulcis_. This preparation is generally made in the dry way, by
triturating equal parts of mercury, common salt and vitriol, and
exposing the whole to a moderate heat; when the corrosive sublimate
rises, and adheres to the upper part of the glass vessel in which the
process is made.

Mercury combines readily with sulphur by trituration, and with it forms
a black powder called _Ethiops mineral_. A more intimate combination of
mercury and sulphur is made by means of fire. This is called _cinnabar_,
about one third of which is sulphur. Vermillion is cinnabar reduced to
powder.

Mercury readily unites with oil, and with it forms a deep black or blue
compound, used in medicine.

It readily combines with most of the metals, and when it is used in a
sufficient quantity to make them soft, the mixture is called an
_amalgam_. It combines most readily with gold, silver, lead, tin,
bismuth, and zinc. Looking-glasses are covered on the back with an
amalgam of mercury and tin.

When mercury is united with lead or other metals, it is rendered less
brilliant and less fluid; but agitation in pure air converts the impure
metal into a calx, together with much of the mercury, in the form of a
black powder.

Heat recovers the pure air, and the mercury, leaving the calx of the
impure metal.

Much mercury is found native in a slaty kind of earth, or in masses of
clay or stone; but the greatest quantity is found combined with sulphur
in _native cinnabar_.




LECTURE XXIV.


_Of Lead._

Lead is a metal of a bluish tinge, of no great tenacity, but very
considerable specific gravity, being heavier than silver. It melts long
before it is red hot, and is then calcined, if it be in contact with
respirable air. When boiling it emits fumes, and calcines very rapidly.
It may be granulated by being poured into a wooden box, and agitated.
During congelation it is brittle, so that the parts will separate by the
stroke of a hammer; and by this means the form of its crystals may be
discovered.

In the progress of calcination it first becomes a dusky grey powder,
then yellow, when it is called _massicot_; then, by imbibing pure air,
it becomes red, and is called _minium_, or _red lead_. In a greater
degree of heat it becomes massicot again, having parted with its pure
air. If the heat be too great, and applied rapidly, it becomes a flaky
substance, called _litharge_; and with more heat it becomes a _glass_,
which readily unites with metallic calces and earths, and is a principal
ingredient in the manufacture of _flint glass_, giving it its peculiar
density and refractive power.

Though lead soon tarnishes, the imperfect calx thus made does not
separate from the rest of the metal, and therefore protects it from any
farther action of the air, by which means it is very useful for the
covering of houses, and other similar purposes. All acids act upon lead,
and form with it different saline substances. _White-lead_ consists of
its union with vinegar and pure air. Also dissolved in vinegar, and
crystallized, it becomes _sugar of lead_, which, like all the other
preparations of this metal, is a deadly poison.

Oils dissolve the calces of lead, which, by this means, is the basis of
paints, plaisters, &c.

By means of heat litharge decomposes common salt, the lead uniting with
the marine acid, and forming a yellow substance, used in painting, and
by this means the fossil alkali is separated.

Lead unites with most metals, though not with iron. Two parts of lead
and one of tin make a _solder_, which melts with less heat than either
of the metals separately; but a composition of eight parts of bismuth,
five of lead, and three of tin, makes a metal which melts in boiling
water.

This metal will be dissolved by water if it contain any saline matter,
and the drinking of it occasions a peculiar kind of cholic.

Lead is sometimes found native, but generally minerally mineralized with
sulphur or arsenic, and often mixed with a small quantity of silver.


_Of Copper._

Copper is a metal of a reddish or brownish colour, considerably
sonorous, and very malleable.

At a heat far below ignition, the surface, of copper becomes covered
with a range of prismatic colours, the commencement of its calcination;
and with more heat a black scale is formed, which easily separates from
the metal, and in a strong heat it melts, and burns with a bluish green
flame.

Copper rusts by exposure to the air; but the partially-calcined surface
adheres to the metal, as in the case of lead, and thus preserves it from
farther corrosion.

Copper dissolved in the vitriolic acid forms crystals of a blot colour,
called _blue copperas_. From this solution it is precipitated by iron,
which by this means becomes coated with copper. The nitrous acid
dissolves copper with most rapidity, producing nitrous air. If the
solution be distilled, almost all the acid will be retained in the
residuum, which is white; but more heat will expel the acid, chiefly in
the form of dephlogisticated air, and the remainder will be a black
substance, consisting of the pure calx of copper. The vegetable acids
dissolve copper as well as the mineral ones, which makes the use of this
metal for culinary purposes in some cases dangerous. To prevent this
they give it a coat of tin. The solution of copper in the vegetable acid
is called _verdigris_.

Alkalies dissolve copper as well as acids. With the volatile alkali a
blue liquor is formed, but in some cases it becomes colourless. All the
circumstances of this change of colour have not yet been examined. Both
oil and sulphur will dissolve copper, and with the latter it forms a
blackish grey compound, used by dyers.

Copper readily unites with melted tin, at a temperature much lower than
that which is necessary to melt the copper; by which means copper
vessels are easily covered with a coating of tin. A mixture of copper
and tin, called _bronze_, the specific gravity of which is greater than
that of the medium of the two metals, is used in casting statues,
cannon, and bells; and in a certain proportion this mixture is excellent
for the purpose of mirrors of reflecting telescopes, receiving a fine
polish, and not being apt to tarnish. Copper and arsenic make a brittle
compound called _tombach_; and with zinc it makes the useful compound
commonly called _brass_, in which zinc is about one third of its weight.

Copper is sometimes found native; but commonly mixed with sulphur, in
ores of a red, green, or blue colour.

Copper being an earlier discovery than that of iron, was formerly used
for weapons and the shoeing of horses; and the ancients had a method,
with which we are not well acquainted, of giving it a considerable
degree of hardness, so that a sword made of it might have a pretty good
edge.




LECTURE XXV.


_Of Iron._

Iron is a metal of a bluish colour, of the greatest hardness, the most
variable in its properties, and the most useful of all the metals; so
that without it it is hardly possible for any people to make great
advances in arts and civilization.

This metal readily parts with its phlogiston, so as to be very subject
to calcine, or rust, by exposure to the air. The same is evident by the
colours which appear on its surface when exposed to heat, and also when
it is struck with flint; the particles that fly from it being iron
partially calcined. In consequence of its readily parting with its
phlogiston, it is capable of burning, like wood or other fuel, in pure
air.

Iron and platina have the property of _welding_ when very hot, so that
two pieces may be joined without any solder.

When iron is heated in contact with steam, part of the water takes the
place of the phlogiston, while the rest unites with it, and makes
inflammable air. By this means the iron acquires one third more weight,
and becomes what is called _finery cinder_. This substance, heated in
inflammable air, imbibes it, parts with its water, and becomes perfect
iron again. If the iron be heated in pure air, it also imbibes the
water, of which that air chiefly consists, and also a portion of the
peculiar element of the pure air.

The solution of iron in spirit of vitriol produces _green copperas_;
which being calcined, becomes a red substance, called _colcothar_.

The precipitate of iron, by an infusion of galls, is the colouring
matter in _ink_, which is kept suspended by means of gum. The
precipitate from the same solution by phlogisticated alkali, is
_Prussian blue_.

Water saturated with fixed air dissolves iron, and makes a pleasant
chalybeat.

The calx of iron gives a green colour to glass.

Iron readily combines with sulphur. When they are found combined by
nature, the substance is called _pyrites_.

The union of phosphoric acid with iron makes it brittle when cold,
commonly called _cold short_; and the union of arsenic makes it brittle
when hot, thence called _red short_.

Iron unites with gold, silver, and platina, and plunged in a white heat
into mercury, it becomes coated with it; and if the process be
frequently repeated, it will become brittle, which shews that there is
some mutual action between them.

Iron readily unites with tin; and by dipping thin plates of iron into
melted tin, they get a complete coating of it, and make the _tinned
plates_ in common use.

When crude iron comes from the smelting furnace it is brittle; and when
it is white within, it is extremely hard; but when it has a black grain,
owing to its having more phlogiston, it is softer, and may be filed and
bored.

Cast iron becomes _malleable_ by being exposed to a blast of air when
nearly melting; the consequence of which is a discharge of inflammable
air, and the separation of a liquid substance, which, when concreted, is
called _finery cinder_. The iron generally loses one fourth of its
weight in the process. Crude iron contains much _plumbago_, and the
access of pure air probably assists in discharging it, by converting it
into air, chiefly inflammable.

Malleable iron, exposed to a red heat in contact with charcoal, called
_cementation_, converts it into _steel_, which has the properties of
becoming much harder than iron, and very elastic, by being first made
very hot, and then suddenly cooled, by plunging it in cold water. By
first making it very hard, and then giving different degrees of heat,
and cooling it in those different degrees, it is capable of a great
variety of _tempers_, proper for different uses. Of the degrees of heat
workmen judge by the change of colour on its surface. Steel, like crude
iron, is capable of being melted without losing its properties. It is
then called _cast steel_, and is of a more uniform texture. Iron
acquires some little weight by being converted into steel; and when
dissolved in acid, it yields more plumbago. Steel has something less
specific gravity than iron. If the cementation be continued too long,
the steel acquires a darkish fracture, it is more fusible, and incapable
of welding. Steel heated in contact with earthy matters, is reduced to
iron.

Iron is the only substance capable of _magnetism_; and hardened steel
alone is capable of retaining magnetism. The loadstone is an ore of
iron.




LECTURE XXVI.


_Of Tin._

Tin is a metal of a slightly yellowish cast, though harder than lead,
very malleable, but of no great tenacity; so that wires cannot be made
of it. It easily extends under the hammer, and plates of it, called
_tinfoil_, are made only one thousandth part of an inch thick, and might
be made as thin again.

Tin has less specific gravity than any other metal. It melts long before
ignition, at 410 of Fahrenheit, and by the continuance of heat is slowly
converted into a white powder, which is the chief ingredient in _putty_,
used in polishing, &c. Like lead, it is brittle when heated little short
of fusion, and may be reduced into grains by agitation as it passes from
a fluid to a solid state.

The calx of tin resists fusion more than that of any other metal, which
makes it useful in making an opaque white enamel.

Tin loses its bright surface when exposed to the air, but is not
properly subject to rust; so that it is useful in protecting iron and
other metals from the effects of the atmosphere.

Concentrated vitriolic acid, assisted by heat, dissolves half its weight
of tin, and yields vitriolic acid air. With more water it yields
inflammable air. During the solution the phlogiston of the tin uniting
with the acid, forms sulphur, which makes it turbid. By long standing,
or the addition of water, the calx of tin is precipitated from the
solution. The nitrous acid dissolves tin very rapidly without heat, and
yields but little nitrous air. With marine acid this metal yields
inflammable air. With aqua regia it assumes the form of a gelatinous
substance used by dyers to heighten the colour of some red tinctures, so
as to produce a bright scarlet in dying wool.

A transparent liquor, which emits very copious fumes, called, from the
inventor, _the smoking liquor of Libavius_, is made by distilling equal
parts of amalgam of tin and mercury with corrosive sublimate, triturated
together. A colourless liquor comes over first, and then a thick white
fume, which condenses into the transparent liquor above mentioned. Mr.
Adet has shewn, that this liquor bears the same relation to the common
solution of tin, that corrosive sublimate does to calomel, and has given
an ingenious solution of many of its properties.

Tin detonates with nitre; and if the crystals made by the solution of
copper in the nitrous acid be inclosed in tinfoil, nitrous fumes will be
emitted, and the whole will become red hot. Also if five times its
weight of sulphur be added to melted tin, a black brittle compound,
which readily takes fire, will be formed.

Another combination of tin, sulphur, and mercury, makes a light yellow
substance called _aurum musivum_ used in painting.

Tin is the principal ingredient in the composition of _pewter_, the
other ingredients being lead, zinc, bismuth, and copper; each pewterer
having his peculiar receipt. It is also used in coating copper and iron
plates, and in silvering looking-glasses, besides being cast into a
variety of forms, when it is called _block tin_.

Tin is sometimes found native, but is generally mineralized with sulphur
and arsenic. The latter is thought to be always contained in tin, and to
be the cause of the crackling noise made by bending plates of it.


_Of the Semi-metals._

Bismuth is a semi-metal of a yellowish or reddish cast, but little
subject to change in the air; harder than lead, but easily broken, and
reducible to powder. When broken it exhibits large shining facets, in a
variety of positions. Thin pieces of it are considerably sonorous.

Bismuth melts at about 460 deg. of Fahrenheit. With more heat it ignites,
and burns with a slight blue flame, while a yellowish calx, called
_flowers of bismuth_, is produced. With more heat it becomes a greenish
glass. In a strong heat, and in close vessels, this metal sublimes.

Vitriolic acid, even concentrated and boiling, has but little effect
upon bismuth; but the nitrous acid acts upon it with the greatest
rapidity and violence, producing much nitrous air, mixed with
phlogisticated nitrous vapour. From the solution of bismuth in this
acid, a white substance, called _magistery of bismuth_, is precipitated
by the affusion of water. This has been used as a paint for the skin
but has been thought to injure it.

The marine acid does not readily act upon bismuth; but when
concentrated, it forms with it a saline combination, which does not
easily crystallize, but may be sublimed in the form of a soft fusible
salt, called _butter of bismuth_.

Bismuth unites with most metallic substances, and in general renders
them more fusible. When calcined with the imperfect metals, it unites
with them, and has the same effect as lead in cupellation.

Bismuth is used in the composition of pewter, in printers' types, and
other metallic mixtures.

This metal is sometimes found native, but more commonly mineralized with
sulphur.




LECTURE XXVII.


_Of Nickel._

Nickel is a semi-metal of a reddish cast, of great hardness, and always
magnetical; on which account it is supposed to contain iron, though
chemists have not yet been able to separate them.

The purest nickel was so infusible as not to run into a mass in the
strongest heat of a smith's forge; but then it was in some degree
malleable.

Concentrated acid of vitriol only corrodes nickel. Alkalies precipitate
it from its solution in the nitrous acid, and dissolve the precipitate.
It readily unites with sulphur.

Nickel is found either native or mineralized with several other metals,
especially with copper, when it is called _kupfer nickel_, or _false
copper_, being of a reddish or copper colour.

This semi-metal has not yet been applied to any use.


_Of Arsenic._

What is commonly called _arsenic_ is the calx of a semi-metal called the
_regulus of arsenic_. It is a white and brittle substance, expelled from
the ores of several metals by heat. It is then refined by a second
sublimation, and melted into the masses in which it is commonly sold.
This calx is soluble in about eighty times its weight of cold water, or
in fifteen times its weight of boiling water. It acts in many respects
like an acid, as it decomposes nitre by distillation, when the nitrous
acid flies off, and the _arsenical salt of Macquer_ remains behind.

When the calx of arsenic is distilled with sulphur, the vitriolic acid
flies off, and a substance of a yellow colour, called _orpiment_, is
produced. This appears to consist of sulphur and the regulus of arsenic;
part of the sulphur receiving pure air from the calx, to which it
communicates phlogiston; and thus the sulphur becomes converted into
vitriolic acid, while the arsenical calx is reduced, and combines with
the rest of the sulphur.

The combination of sulphur and arsenic, by melting them together, is of
a red colour, known by the name of _realgal_, or _realgar_. It is less
volatile than orpiment.

The solution of fixed alkali dissolves the calx of arsenic, and by means
of heat a brown tenacious mass is produced, and having also a
disagreeable smell, it is called _liver of arsenic_.

The regulus of arsenic is of a yellow colour, subject to tarnish or grow
black, by exposure to the air, very brittle, and of a laminated texture.
In close vessels it wholly sublimes, but burns with a small flame in
pure air.

Vitriolic acid has little action upon this semi-metal, except when hot;
but the nitrous acid acts readily upon it, and likewise dissolves the
calx, as does boiling marine acid, though it affects it very little when
cold.

Most of the metals unite with the regulus of arsenic.

Dephlogisticated marine acid converts the calx of arsenic into
_arsenical acid_ by giving it pure air.

The acid of arsenic acts more or less upon all metals, but the phenomena
do not appear to be of much importance.

The calx of acid is used in a variety of the arts, especially in the
manufactory of glass. Orpiment and realgar are used as pigments. Some
attempts have been made to introduce it into medicine, but being
dangerous, the experiments should be made with caution.


_Of Cobalt._

Cobalt is a semi-metal of a grey or steel colour, of a close-grained
fracture, more difficult of fusion than copper, not easily calcined. It
soon tarnishes in the air, but water has no effect upon it.

Cobalt, dissolved in _aqua regia_, makes an excellent sympathetic ink,
appearing green when held to the fire, and disappearing when cold,
unless it has been heated too much, when it burns the paper.

The calx of cobalt is of a deep blue colour, which, when fused, makes
the blue glass called _smalt_. The ore of cobalt, called _zaffre_, is
found in several parts of Europe, but chiefly in Saxony. As it is
commonly sold, it contains twice or thrice its weight of powder of
flints. The smalt is usually composed of one part of calcined cobalt,
fused with two parts of powder of flint and one of pot-ash.

The chief use of cobalt is for making smalt; but the powder and the
blue-stone used by laundresses is a preparation made by the Dutch of a
coarse kind of smalt.


_Of Zinc._

Zinc is a semi-metal of a bluish cast, brighter than lead, and so far
malleable as not to be broken by a hammer, though it cannot be much
extended. When broken by bending, it appears to consist of cubical
grains. If it be heated nearly to melting, it will be sufficiently
brittle to be pulverized. It melts long before ignition, and when it is
red hot, it burns with a dazzling white flame, and is calcined with
such rapidity, that its calx flies up in the form of white flowers,
called _flowers of zinc_, or _philosophical wool_. In a stronger heat
they become a clear yellow glass. Heated in close vessels, this metal
rises without decomposition, being the most volatile of all the metals
except the regulus of _arsenic_.

Zinc dissolved in diluted vitriolic acid, yields much inflammable air,
and has a residuum, which appears to be plumbago, and the liquor forms
crystals, called _white copperas_. This metals also yields inflammable
air when dissolved in the marine acid. Dissolved in the nitrous acid, it
yields dephlogisticated nitrous air, with very little proper nitrous
air.

The ore of zinc, called _calamine_, is generally of a white colour; and
the chief use of it is to unite it with copper, with which it makes
brass and other gold- mixtures of metals. The calx and the salts
of this metal are occasionally used in medicine.




LECTURE XXVIII.


_Of Antimony._

The regulus of antimony is of a silvery white colour, of a scaly
texture, very brittle, and melts soon after ignition. By continuance of
heat it calcines in white fumes, called _argentine flowers of antimony_,
which melt into a hyacinthine glass. In close vessels it rises without
decomposition. Its calx is soluble in water, like that of arsenic. This
metal tarnishes, but does not properly rust, by exposure to the air.

This metal is soluble in aqua regia. It detonates with nitre, and what
remains of equal parts of nitre and regulus of antimony after
detonation, in a hot crucible, is called _diaphoretic antimony_. The
water used in this preparation contains a portion of the calx suspended
by the alkali, and being precipitated by an acid, is called _ceruse of
antimony_.

When regulus of antimony is pulverized and mixed with twice its weight
of corrosive sublimate (which is attended with heat) and then distilled
with a gentle fire, a thick fluid comes over, which is congealed in the
receiver, or in the neck of the retort, and is called _butter of
antimony_. The residuum consists of revived mercury, with some regulus
and calx of antimony. When this butter of antimony is thrown into pure
water, there is a white precipitate, called _powder of algaroth_, a
violent emetic. Nitrous acid dissolves the butter of antimony; and when
an equal weight of nitrous acid has been three times distilled to
dryness from butter of antimony, the residuum, after ignition, is called
_bezoar mineral_, and seems to be little more than a calx of the metal.

Crude antimony, which has been much used in the experiments of
alchemists, is a combination of sulphur and regulus of antimony. Heat
melts it, and finally converts it into glass, of a dark red colour,
called _liver of antimony_. If antimony be melted or boiled with a fixed
alkali, a precipitate is made by cooling, called _kermes mineral_,
formerly used in medicine. The antimonial preparations that are now most
in use are _antimonial wine_ and _tartar emetic_. The wine is made by
infusing pulverized glass of antimony in Spanish wine some days, and
filtering the clear fluid through paper. The emetic tartar, or
antimonial tartar, is a saline substance, composed of acid of tartar,
vegetable alkali, and antimony partially calcined. The preparation may
be seen in the Dispensaries.

The regulus of antimony is used in the form of pills, which purge more
or less in proportion to the acid they meet with; and as they undergo
little or no change in passing through the body, they are called
_perpetual pills_.


_Of Manganese._

Manganese is a hard, black mineral, very ponderous, and the regulus of
it is a semi-metal of a dull white colour when broken, but soon grows
dark by exposure to the air. It is hard and brittle, though not
pulverizable, rough in its fracture, and of very difficult fusion. Its
calces are white when imperfect, but black, or dark green, when perfect.
The white calx is soluble in acids. When broken in pieces, it falls into
powder by a spontaneous calcination, and this powder is magnetical,
though the mass was not possessed of that property. The black calx of
manganese is altogether insoluble in acids. It contains much
dephlogisticated air.

The calx of manganese is used in making glass; the glass destroying the
colour of that of the other materials, and thereby making the whole mass
transparent.

This semi-metal mixes with most of the metals in fusion, but not with
mercury.

There is another ore of manganese, called _black woad_, which inflames
spontaneously when mixed with oil.


_Of Wolfram._

Wolfram is a mineral of a brownish or black colour, found in the tin
mines of Cornwall, of a radiated or foliated texture, shining almost
like a metal. It contains much of the calx of manganese, and iron; but
when the substance is pulverized, these are easily dissolved, and the
calx of wolfram is found to be yellow.

This calx turns blue by exposure to light; and an hundred grains of it
heated with charcoal will yield sixty grains of a peculiar metal, in
small particles, which, when broken, look like steel. It is soluble in
the vitriolic or marine acids, and reduced to a yellow calx by nitrous
acid or aqua regia.


_Of Molybdena._

Molybdena is a substance which much resembles plumbago; but its texture
is scaly, and not easily pulverized, on account of a degree of
flexibility which its laminae possess. With extreme heat, and mixed with
charcoal, it yields small particles of a metal that is grey, brittle,
and extremely infusible; and uniting with several of the metals, it
forms with them brittle or friable compounds. By heat it is converted
into a white calx.


_Of Solid Combustible Substances._

There yet remains a class of solid substances, of the _combustible_
kind, but most of them have been already considered under the form of
the fluids, from which they are originally formed, as _bitumen_,
_pit-coal_, and _amber_; or under the principal ingredients of which
they are composed, as _sulphur_ and _plumbago_.

There only remains to be mentioned the _diamond_, which is of a nature
quite different from that of the other precious stones, the principal
ingredient in which is siliceous earth, which renders them not liable to
be much affected by heat. On the contrary, the diamond is a combustible
substance; for in a degree of heat somewhat greater than that which will
melt silver, it burns with a slight flame, diminishes common air, and
leaves a soot behind. Also, if diamond powder be triturated with
vitriolic acid, it turns it black, which is another proof of its
containing phlogiston.

The diamond is valued on account of its extreme hardness, the exquisite
polish it is capable of, and its extraordinary refractive power; for
light falling on its interior surface with an angle of incidence greater
than 24-1/2 will be wholly reflected, whereas in glass it requires an angle
of 41 degrees.




LECTURE XXIX.


_Of the Doctrine of Phlogiston and the Composition of Water._

It was supposed to be a great discovery of Mr. Stahl, that all
inflammable substances, as well as metals, contain a principle, or
substance, to which he gave the name of phlogiston, and that the
addition or deprivation of this substance makes some of the most
remarkable changes in bodies, especially that the union of a metallic
calx and this substance makes a metal; and that combustion consists in
the separation of phlogiston from the substances that contain it. That
it is the same principle, or substance, that enters into all inflammable
substances, and metals, is evident, from its being disengaged from any
of them, and entering into the composition of any of the others. Thus
the phlogiston of charcoal or inflammable air becomes the phlogiston of
any of the metals, when the calx is heated in contact with either of
them.

On the contrary, Mr. Lavoisier and most of the French chemists, are of
opinion, that there is no such principle, or substance, as phlogiston;
that metals and other inflammable bodies are simple substances, which
have an affinity to pure air; and that combustion consists not in the
separation of any thing from the inflammable substance, but in the union
of pure air with it.

They moreover say, that water is not, as has been commonly supposed, a
simple substance, but that it consists of two elements, viz. pure air,
or _oxygene_, and another, to which they give the name of _hydrogene_,
which, with the principle of _heat_, called by them _calorique_, is
inflammable air.

The principal fact adduced by them to prove that metals do not lose any
thing when they become calces, but only gain something, is, that mercury
becomes a calx, called _precipitate per se_, by imbibing pure air, and
that it becomes running mercury again by parting with it.

This is acknowledged: but it is almost the only case of any calx being
revived without the help of some known phlogistic substance; and in this
particular case it is not absurd to suppose, that the mercury, in
becoming precipitate per se, may retain all its phlogiston, as well as
imbibe pure air, and therefore be revived by simply parting with that
air. In many other cases the same metal, in different states, contains
more or less phlogiston, as cast iron, malleable iron, and steel. Also
there is a calx of mercury made by the acid of vitriol, which cannot be
revived without the help of inflammable air, or some other substance
supposed to contain phlogiston: and that the inflammable air is really
imbibed in these processes, is evident, from its wholly disappearing,
and nothing being left in the vessel in which the process is made beside
the metal that is revived by it. If precipitate per se be revived in
inflammable air, the air will be imbibed, so that running mercury may
contain more or less phlogiston.

The antiphlogistians also say, that the diminution of atmospherical air
by the burning of phosphorus is a proof of their theory; the pure air
being imbibed by that substance, and nothing emitted from it. But there
is the same proof of phosphorus containing phlogiston, that there is of
dry flesh containing it; since the produce of the solution of it in
nitrous acid, and its effect upon the acid, are the same, viz. the
production of phlogisticated air, and the phlogistication of the acid.

Their proof that water is decomposed, is, that in sending steam over hot
iron, inflammable air (which they suppose to be one constituent part of
it) is procured; while the other part, viz. the oxygene, unites with the
iron, and adds to its weight. But it is replied, that the inflammable
air may be well supposed to be the phlogiston of the iron, united to
part of the water, as its base, while the remainder of the water is
imbibed by the calx; and that it is mere water, and not pure air, or
oxygene, that is retained in the iron, is evident, from nothing but pure
water being recovered when this calx of iron is revived in inflammable
air, in which case the inflammable air wholly disappears, taking the
place of the water, by which it had been expelled.

In answer to this it is said, that the pure air expelled from the calx
uniting with the inflammable air in the vessel, recomposes the water
found after this process. But in every other case in which any substance
containing pure air is heated in inflammable air, though the inflammable
air be in part imbibed, some _fixed air_ is produced, and this fixed air
is composed of the pure air in the substance and part of the inflammable
air in the vessel. Thus, if _minium_, which contains pure air, and
_massicot_, which contains none, be heated in inflammable air, in both
the cases lead will be revived by the absorption of inflammable air; but
in the former case only, and not in the latter, will fixed air be
produced. The calx of iron, therefore, having the same effect with
massicot, when treated in the same manner, appears to contain no more
pure air than massicot does.

Besides this explanation of the facts on which the new theory is
founded, which shews it to be unnecessary, the old hypothesis being
sufficient for the purpose, some facts are alledged, as inconsistent
with the new doctrine.

If the calx of iron made by water, and charcoal made by the greatest
degree of heat, be mixed together, a great quantity of inflammable air
will be produced; though, according to the new theory, neither of these
substances contained any water, which they maintain to be the only
origin of it. But this fact is easily explained upon the doctrine of
phlogiston; the water in this calx uniting with the phlogiston of the
charcoal, and then forming inflammable air; and it is the same kind of
inflammable air that is made from charcoal and water.

Also the union of inflammable and pure air, when they are fired together
by means of the electric spark, produces not pure water, as, according
to the new theory, it ought to do, but _nitrous acid_.

To this it has been objected, that the acid thus produced came from the
decomposition of phlogisticated air, a small portion of which was at
first contained in the mixture of the two kinds of air. But when every
particle of phlogisticated air is excluded, the strongest acid is
procured.

They find, indeed, that by the slow burning of inflammable air in pure
air, they get pure water. But then it appears, that whenever this is the
case, there is a production of phlogisticated air, which contains the
necessary element of nitrous acid; and this is always the case when
there is a little surplus of the inflammable air that is fired along
with the pure air, as the acid is always procured when there is a
redundancy of pure air.

That much water should be procured by the decomposition of these kinds
of air, is easily accounted for, by supposing that water, or steam, is
the basis of these, as well as of all other kinds of air.

Since air something better than that of the atmosphere is constantly
produced from water by converting it into vapour, and also by removing
the pressure of the atmosphere, and these processes do not appear to
have any limits; it seems probable, that _water_ united to the principle
of _heat_; constitutes atmospherical air; and if so, it must consist of
the elements of both dephlogisticated and phlogisticated air; which is a
supposition very different from that of the French chemists.




LECTURE XXX.


_Of Heat._

Heat is an affection of bodies well known by the sensation that it
excites. It is produced by friction or compression, as by the striking
of flint against steel, and the hammering of iron, by the reflection or
refraction of light, and by the combustion of inflammable substances.

It has been long disputed, whether the cause of heat be properly a
_substance_, or some particular affection of the particles that compose
the substance that is heated. But be it a substance, or a principle of
any other kind, it is capable of being transferred from one body to
another, and the communication of it is attended with the following
circumstances. All substances are expanded by heat, but some in a
greater degree than others; as metals more than earthy substances, and
charcoal more than wood. Also some receive and transmit heat through
their substance more readily than others; metals more so than earths,
and of the metals, copper more readily than iron. Instruments contrived
to ascertain the expansion of substances by heat, are called
_pyrometers_, and are of various constructions.

As a standard to measure the degrees of heat, mercury is in general
preferable to any other substance, on account of its readily receiving,
and communicating, heat through its whole mass. _Thermometers_,
therefore, or instruments to measure the degrees of heat, are generally
constructed of it, though, as it is subject to become solid in a great
degree of cold, ardent spirit, which will not freeze at all, is more
proper in that particular case.

The graduation of thermometers is arbitrary. In that of Fahrenheit,
which is chiefly used in England, the freezing point of water is 32 deg.,
and the boiling point 212 deg. In that of Reaumur, which is chiefly used
abroad, the freezing point of water is 0, and the boiling point 80. To
measure the degrees of heat above ignition, Mr. Wedgwood has happily
contrived to use pieces of clay, which contract in the fire; and he has
also been able to find the coincidence of the degrees in mercurial
thermometers with those of his own.

To measure the degrees of heat and cold during a person's absence, Lord
George Cavendish contrived an instrument, in which a small bason
received the mercury, that was raised higher than the place for which it
was regulated by heat or cold, without a power of returning. But Mr. Six
has lately hit upon a better method, viz. introducing into the tube of
his thermometer a small piece of iron, which is raised by the ascent of
the mercury, and prevented from descending by a small spring; but which
may be brought back to its former place by a magnet acting through the
glass.

Heat, like light, is propagated in right lines; and what is more
remarkable, cold observes the same laws. For if the substance emitting
heat without light, as iron below ignition, be placed in the focus of a
burning mirror, a thermometer in the focus of a similar mirror, placed
parallel to it, though at a considerable distance, will be heated by it,
and if a piece of ice be placed there, the mercury will fall.

Heat assists the solvent power of almost all menstrua; so that many
substances will unite in a certain degree of heat, which will form no
union at all without it, as dephlogisticated and inflammable air.

If substances be of the same kind, they will receive heat from one
another, in proportion to their masses. Thus, if a quantity of water
heated to 40 deg. be mixed with another equal quantity of water heated to
20 deg., the whole mass will be heated to 30 deg. But if the substances be of
different kinds, they will receive heat from each other in different
proportions, according to their _capacity_ (as it is called) of
receiving heat. Thus, if a pint of mercury of the temperature of 136 be
mixed with a pint of water of the temperature of 50, the temperature of
the two after mixture will not be a medium between those two numbers,
viz. 93, but 76; consequently the mercury was cooled 60 deg., while the
water was heated only 26; so that 26 degrees of heat in water correspond
to 60 in mercury. But mercury is about 13 times specifically heavier
than water, so that an equal weight of mercury would contain only one
thirtieth part of this heat; and dividing 26 by 13, the quotient is 2.
If _weight_, therefore, be considered, the heat discovered by water
should be reckoned as 2 instead of 60; and consequently when water
receives 2 degrees of heat, an equal weight of mercury will receive 60 deg.;
and dividing both the numbers by 2, if the heat of water be 1, that of
the mercury will be 30. Or since they receive equal degrees of heat,
whether they discover it or not (and the less they discover, the more
they retain in a latent state) a pound of mercury contains no more than
one thirtieth part of the heat actually existing in a pound of water of
the same temperature. Water, therefore, is said to have a greater
capacity for receiving and retaining heat, without discovering it, than
mercury, in the proportion of 30 to 1, if weight be considered, or of
60 to 26, that is of 30 to 13; if _bulk_ be the standard, though,
according to some, it is as 3 to 2.

The capacity of receiving heat in the substance is greatest in a state
of vapour, and least in that of a solid; so that when ice is converted
into water, heat is absorbed, and more still when it is converted into
vapour; and on the contrary, when vapour is converted into water, it
gives out the heat which it had imbibed, and when it becomes ice it
gives out still more.

If equal quantities of ice and water be exposed to heat at the
temperature of 32 deg., the ice will only become water, without receiving
any additional sensible heat; but an equal quantity of water in the same
situation would be raised to 178 deg., so that 146 degrees of heat will be
imbibed, and remain in latent in the water, in consequence of its
passing from a state of ice: and heat communicated by a given weight of
vapour will raise an equal weight of a nonevaporable substance, of the
same capacity with water, 943 degrees; so that much more heat is latent
in steam, than in the water from which it was formed.

This doctrine of latent heat explains a great variety of phaenomena in
nature; as that of cooling bodies by evaporation, the vapour of water,
or any other fluid substance, absorbing and carrying off the heat they
had before.

Water, perfectly at rest, will fall considerably below the freezing
point, and yet continue fluid: but on the slightest agitation, the
congelation of the whole, or part of it, takes place instantly, and if
the whole be not solid, it will instantly rise to 32 deg., the freezing
point. From whatever cause, some motion seems necessary to the
commencement of congelation, at least in a moderate temperature; but
whenever any part of the water becomes solid, it gives out some of the
heat it had before, and that heat which was before latent becoming
sensible, and being diffused through the whole mass, raises its
temperature.

On the same principle, when water heated higher than the boiling point
in a digester is suddenly permitted to escape in the form of steam, the
remainder is instantly reduced to the common boiling point, the heat
above that point being carried off in a latent state by the steam.

Had it not been for this wise provision in nature, the whole of any
quantity of water would, in all cases of freezing, have become solid at
once; and also the whole of any quantity that was heated to the point of
boiling, would have been converted into steam at once; circumstances
which would have been extremely inconvenient, and often fatal.

This doctrine also explains the effect of freezing mixtures, as that of
salt and snow. These solid substances, on being mixed, become fluid, and
that fluid absorbing much heat, deprives all the neighbouring bodies of
part of what they had. But if the temperature at which the mixture is
made be as low as that to which this mixture would have brought it, it
has no effect, and in a lower temperature this new fluid would become
solid; for that mixture has only a certain determinate capacity for
heat, and if the neighbouring bodies have less heat, they will take
from it.

It has been observed, that the comparative heat of bodies containing
phlogiston is increased by calcination or combustion; so that the calx
of iron has a greater capacity for heat, and therefore contains more
latent heat, than the metal.

In general it is not found, that the same substances have their capacity
for receiving heat increased by an increase of temperature; but this is
said to be the case with a mixture of ardent spirit and water, and also
that of spirit of vitriol and water.

Since all substances contain a greater or less quantity of heat, and in
consequence of being deprived of it become colder and colder, it is a
question of some curiosity to determine the extent to which this can go,
or at what degree in the scale of a thermometer any substance would be
absolutely cold, or deprived of all heat; and an attempt has been made
to solve this problem in the following manner. Comparing the capacity of
water with that of ice, by means of a third substance, viz. mercury, it
has been found, that if that of ice be 9 deg., that of water is 10 deg.; so that
water in becoming ice gives out one tenth part of its whole quantity of
heat. But it has been shown, that ice in becoming water absorbs 146
degrees of heat. This, therefore, being one tenth part of the whole heat
of water, it must have contained 1460 degrees; so that taking 32
degrees, which is the freezing point, from that number, the point of
absolute cold will be 1426 below 0 of Fahrenheit's scale.

By a computation, made by means of the heat of inflammable and
dephlogisticated air, at the temperature of 50, Dr. Crawford finds, that
it contains nearly 1550 degrees of heat; so that the point of absolute
cold will be 1500 below 0. But more experiments are wanted to solve this
curious problem to entire satisfaction.




LECTURE XXXI.


_Of Animal Heat._

Since all animals, and especially those that have red blood, are much
hotter than the medium in which they live, the source of this heat has
become the subject of much investigation; and as the most probable
theory is that of Dr. Crawford, I shall give a short detail of the
reasons on which it is founded.

Having, with the most scrupulous attention, ascertained the _latent_,
or, as he calls it, the _absolute_ heat of blood, and also that of the
aliments of which it is composed, he finds that it contains more than
could have been derived from _them_. Also finding that the absolute heat
of arterial blood exceeds that of venous blood, in the proportion of 11-1/2
to 10, he concludes that it derives its heat from the air respired in
the lungs, and that it parts with this _latent_ heat, so that it becomes
sensible, in the course of its circulation, in which it becomes loaded
with phlogiston, which it communicates to the air in the lungs.

That this heat is furnished by the _air_, he proves, by finding, that
that which we inspire contains more heat than that which we expire, or
than the aqueous humor which we expire along with it, in a very
considerable proportion; so that if the heat contained in the pure air
did not become latent in the blood, it would raise its temperature
higher than that of red-hot iron. And again, if the venous blood, in
being converted into arterial blood, did not receive a supply of latent
heat from the air, its temperature would fall from 96 to 104 below 0 in
Fahrenheit's thermometer.

That the heat procured by combustion has the same source, viz. the
dephlogisticated air that is decomposed in the process, is generally
allowed; and Dr. Crawford finds, that when equal portions of air are
altered by the respiration of a Guinea pig, or by the burning of
charcoal, the quantity of heat communicated by the two processes is
nearly equal.

The following facts are also alleged in favour of his theory. Whereas
animals which have much red blood, and respire much, have the power of
keeping themselves in a temperature considerably higher than that of the
surrounding atmosphere, other animals, as _frogs_ and _serpents_, are
nearly of the same temperature with it; and those animals which have the
largest respiratory organs, as birds, are the warmest; also the degree
of heat is in some measure proportionable to the quantity of air that is
respired in a given time, as in violent exercise.

It has been observed, that animals in a medium hotter than the blood
have a power of preserving themselves in the same temperature. In this
case the heat is probably carried off by perspiration, while the blood
ceases to receive, or give out, any heat; and Dr. Crawford finds, that
when an animal is placed in a warm medium the colour of the venous blood
approaches nearer to that of the arterial than when it is placed in a
colder medium; and also, that it phlogisticates the air less than in the
former case; so that in these circumstances respiration has not the same
effect that it has in a colder temperature, in giving the body an
additional quantity of heat; which is an excellent provision in nature,
as the heat is not wanted, but, on the contrary, would prove
inconvenient.




LECTURE XXXII.


_Of Light._

Another most important agent in nature, and one that has a near
connexion with heat, is _light_, being emitted by all bodies in a state
of ignition, and especially by the sun, the great source of light and of
heat to this habitable world.

Whether light consists of particles of matter (which is most probable)
or be the undulation of a peculiar fluid, filling all space, it is
emitted from all luminous bodies in right lines.

Falling upon other bodies, part of the light is _reflected_ at an angle
equal to that of its incidence, though not by impinging on the
reflecting surface, but by a power acting at a small distance from it.
But another part of the light enters the body, and is _refracted_ or
bent _towards_, or _from_, the perpendicular to the surface of the new
medium, if the incidence be oblique to it. In general, rays of light
falling obliquely on any medium are bent as if they were attracted by
it, when it has a greater density, or contains more of the inflammable
principle, than the medium through which it was transmitted to it. More
of the rays are reflected when they fall upon a body with a small degree
of obliquity to its surface, and more of them are transmitted, or enter
the body, when their incidence is nearer to a perpendicular.

The velocity with which light is emitted or reflected is the same, and
so great that it passes from the sun to the earth in about eight
minutes and twelve seconds.

Rays of light emitted or reflected from a body entering the pupil of the
eye, are so refracted by the humours of it, as to be united at the
surface of the retina, and so make images of the objects, by means of
which they are visible to us; and the magnifying power of telescopes or
microscopes depends upon contriving, by means of reflections or
refractions, that pencils of rays issuing from every point of any object
shall first diverge, and then converge, as they would have done from a
much larger object, or from one placed much nearer to the eye.

When a beam of light is bent out of its course by refraction, all the
rays of which it consists are not equally refracted, but some of them
more and others less; and the colour which they are disposed to exhibit
is connected invariably with the degree of their refrangibility; the
red- rays being the least, and the violet the most refrangible,
and the rest being more or less so in proportion to their nearness to
these, which are the extremes, in the following order, violet, indigo,
blue, green, yellow, orange, red.

These colours, when separated as much possible, are still contiguous;
and all the shades of each colour have likewise their separate and
invariable degrees of refrangibility. When separated as distinctly as
possible, they divide the whole space between them exactly as a musical
chord is divided in order to found the several notes and half notes of
an octave.

These differently- rays of light are also separated in passing
through the transparent medium of air and water, in consequence of which
the sky appears blue and the sea green, these rays being returned, while
the red ones proceed to a greater distance. By this means also objects
at the bottom of the sea appear to divers red, and so do all objects
enlightened by an evening sun.

The mixture of all the differently- rays, in the proportions in
which they cover the  image above mentioned, makes a _white_,
and the absence of all light is _blackness_.

By means of the different refrangibility of light, the colours of the
rainbow may be explained.

The distance to which the differently- rays are separated from
each other is not in proportion to the mean refractive power of the
medium, but depends upon the peculiar constitution of the substance by
which they are refracted. The _dispersing power_ of glass, into the
composition of which _lead_ enters, is great in proportion to the mean
refraction; and it is proportionally little in that glass in which there
is much alkaline salt. The construction of _achromatic telescopes_
depends upon this principle.

Not only have different rays of light these different properties with
respect to bodies, so as to be more or less refracted, or dispersed, by
them, but different sides of the same rays seem to have different
properties, for they are differently affected on entering a piece of
_island crystal_. With the same degree of incidence; part of the pencil
of rays, consisting of all the colours, proceeds in one direction, and
the rest in a different one; so that objects seen through a piece of
this substance appear double.

At the surface of all bodies rays of light are promiscuously reflected,
or transmitted.

But if the next surface be very near to it, the rays of one colour
chiefly are reflected, and the rest transmitted, and these places occur
alternately for rays of each of the colours in passing from the thinnest
to the thickest parts of the medium; so that several series, or orders,
of colours will be visible on the surface of the same thin transparent
body. On this principle  rings appear between a plane and a
convex lens, in a little oil on the surface of water, and in bubbles
made with soap and water.

When rays of light pass near to any body, so as to come within the
sphere of its attraction and repulsion, an _inflection_ takes place; all
the kinds of rays being bent _towards_, or _from_, the body, and these
powers affecting some rays more than others, they are by this means also
separated from each other, so that  streaks appear both within
the shadow, and the outside of it, the red rays being inflected at the
greatest distance from the body.

Part of the light which enters bodies is retained within them, and
proceeds no farther; but so loosely in some kinds of bodies, that a
small degree of heat is sufficient to expel it again, so as to make the
body visible in the dark: but the more heat is applied, the sooner is
all the light expelled. This is a strong argument for the materiality of
light. _Bolognian phosphorus_ is a substance which has this property;
but a composition made by Mr. Canton, of calcined oyster-shells and
sulphur, in a much greater degree. However, white paper, and most
substances, except the metals, are possessed of this property in a small
degree.

Some bodies, especially phosphorus, and animal substances tending to
putrefaction, emit light without being sensibly hot.

The _colours_ of vegetables, and likewise their _taste_ and _smell_,
depend upon light. It is also by means of light falling on the leaves
and other green parts of plants, that they emit dephlogisticated air,
which preserves the atmosphere fit for respiration.

It is light that imparts colour to the skins of men, by means of the
fluid immediately under them. This is the cause of _tanning_, of the
_copper colour_ of the North Americans, and the _black_ of the <DW64>s.
Light also gives colour to several other substances, especially the
solutions of mercury in acids.




LECTURE XXXIII.


_Of Magnetism._

Magnetism is a property peculiar to iron, or some ores of it. The earth
itself, owing probably to the iron ores contained in it, has the same
property. But though all iron is acted upon by magnetism, _steel_ only
is capable of having the power communicated to it.

Every magnet has two poles, denominated _north_ and _south_, each of
which attracts the other, and repels that of the same kind with itself.
If a magnet be cut into two parts, between the two poles, it will make
two magnets, the parts that were contiguous becoming opposite poles.

Though the poles of a magnet are denominated _north_ and _south_, they
do not constantly, and in all parts of the earth, point due north or
south, but in most places to the east or west of them, and with a
considerable variation in a course of time. Also a magnet exactly
balanced at its center will have a declination from an horizontal
position of about 70 degrees. The former is called the _variation_, and
the latter the _dipping_ of the magnetic needle.

A straight bar of iron which has been long fixed in a vertical position,
will become a magnet, the lower end becoming a north pole, and the upper
end a south one; for if it be suspended horizontally, the lower end will
point towards the north, and the upper end towards the south. Also any
bar of iron, not magnetical, held in a vertical position, will become a
temporary magnet, the lower end becoming a north pole, and the upper end
a south one; and a few strokes of a hammer will fix the poles for a
short time, though the position of the ends be changed. Magnetism may
likewise be given to a bar of iron by placing it firmly in the position
of the dipping-needle, and rubbing it hard one way with a polished steel
instrument. Iron will also become magnetical by ignition and quenching
it in water in the position of the dipping-needle.

Magnetism acts, without any diminution of its force, through any medium;
and iron not magnetical will have that power while it is in connexion
with a magnet, or rather the power of the magnet is extended through the
iron.

Steel filings gently thrown upon a magnet, adhere to it in a curious
manner; and the filings, acquiring magnetism by the contact, adhere
together, and form a number of small magnets, which arrange themselves
according to the attraction of the poles of the original magnet. This
experiment is made to the most advantage upon a piece of pasteboard, or
paper, placed over the magnet.

Magnetism is communicated by the friction, or the near position, of a
magnet to a piece of steel of a size less than it. For this reason a
combination of magnetical bars will have a greater effect than a single
one; and in the following manner, beginning without any magnetism at
all, the greatest quantity may be procured. Six bars of steel may be
rendered slightly magnetical by fixing each of them successively to an
upright poker, and stroking it several times from the bottom to the top
with the lower end of an old pair of tongs. If then four of these bars
be joined, the magnetism of the remaining two will be much increased by
a proper method of rubbing with them; and by changing their places,
joining the strongest, and acting upon the weakest, they may all be made
as magnetical as they are capable of being.

The strength of a natural magnet may be increased by covering its polar
extremities with steel. This is called the _arming_ of the loadstone.

To account for the variation of the needle, Dr. Halley supposed the
earth to consist of two parts, an external _shell_ and an internal
_nucleus_, detached, and having a revolution distinct from it; and that
the action of the poles of the shell and of the nucleus would explain
all the varieties in the position of the needle. But others think that
the cause of the magnetism of the earth is not _within_, but _without_
itself. One reason for this opinion is, that a magnet is liable to be
affected by a strong aurora borealis; and another is, that the variation
of the needle proceeds in such manner as supposes that the motion of the
nucleus must be quicker than that of the shell of the earth; whereas,
since it is most natural to suppose that motion was communicated to the
nucleus by the shell, it would be slower.

Some idea of the quantity and the progress of the variation of the
needle may be formed from the following facts.--At the Cape of Good
Hope, when it was discovered by the Portuguese, in 1486, there was no
variation, the needle there pointing due north; in 1622 it was about 2
degrees westward, in 1675 it was 8 deg. W. in 1700 about 11 deg. W. in 1756
about 18 deg. W. and in 1774 about 21-1/2 deg. W. In London, in 1580, the variation
was 11 degrees 15 seconds E.; in 1622 it was 6 deg. E. in 1634 it was 4 deg.
5 min. E. in 1657 it was nothing at all; in 1672 it was 2 deg. 30 min.
W. in 1692 it was 6 deg. W. in 1753 it was about 16 W. and at present it
is about 21 W.

The longitude may in some places be found by the variation of the
needle; and Mr. Churchman, of America, having given much attention to
the subject, comparing the observations of others, and many of his own,
thinks that he has found a method of determining the longitude to a
great degree of certainty, in most cases, by this means.

He says there are two magnetic poles of the earth, one to the north and
the other to the south, at different distances from the poles of the
earth, and revolving in different times; and from the combined influence
of these two poles he deduces rules for the position of the needle in
all places of the earth, and at all times, past, present, or to come.

The north magnetic pole, he says, makes a complete revolution in 426
years, 77 days, 9 hours, and the south pole in about 5459 years. In the
beginning of the year 1777 the north magnetic pole was in 76 deg. 4 min.
north latitude; and in longitude from Greenwich 140 deg. east; and the
south was in 72 deg. south latitude, and 140 deg. east from Greenwich.




LECTURE XXXIV.


_Of Electricity._

Electricity is a property belonging to, or capable of being communicated
to, all substances whatever; and whereas by some of them it is
transmitted with great ease, and by others with much difficulty, they
have been divided into two classes, and denominated _conductors_ or
_non-conductors_ of electricity. Also the latter receiving this power by
friction, and other means, are termed _electrics_, and the former
_non-electrics_.

Metals of all kinds, and water, are conductors, though in very different
degrees; so also is charcoal. All other substances, and also a perfect
vacuum, are non-conductors of electricity. But many of these substances,
when they are made very hot, as glass, resin, baked wood, and perhaps
all the rest on which the experiment can be made in this state, are
conductors.

It is the property of all kinds of electrics, when they are rubbed by
bodies different from themselves, to attract light substances of all
kinds, to exhibit an appearance of _light_, attended with a particular
_sound_, on the approach of any conductor; and if the nostrils are
presented, they are affected with a _smell_ like that of phosphorus.
This attraction is most easily explained by supposing that electricity
is produced by a fluid exceedingly elastic, or repulsive of itself, and
attracted by all other substances.

An electric exhibiting the appearances above mentioned, is said to be
_excited_, and some of them, particularly the _tourmaline_, are excited
by heating and cooling, as well as by friction. It appears, however,
that excitation consists in the mere transferring of electricity from
one substance to another, and that the great source of electricity is in
the earth. On this account it is necessary to the considerable
excitation of any electric, that the substance against which it is
rubbed (hence termed _the rubber_) have a communication with the earth,
by means of conductors; for if the rubber be _insulated_, that is cut
off from all communication with the earth by means of electrics, the
friction has but little effect.

When insulated bodies have been attracted by, and brought into contact
with, an excited electric, they begin to be repelled by it, and also to
repel one another; nor will they be attracted again till they have been
brought into contact with some conductor communicating with the earth;
but after this they will be attracted as at first.

If conductors be _insulated_, electric powers may be communicated to
them by the approach of excited electrics, or the contact of other
electrified bodies. They will then attract light bodies, and give
sparks, &c. like the excited electrics themselves.

When electricity is strongly communicated to insulated animal bodies,
the pulse is quickened, and perspiration increased; and if they receive,
or part with, their electricity on a sudden, a painful sensation is felt
at the place of communication. But what is more extraordinary, is, that
the influence of the brain and nerves upon the muscles seems to be of an
electric nature.

This is one of the last and most important of all philosophical
discoveries. I shall, therefore, give the result of all the observations
that have hitherto been made on the subject, in a _series of
propositions_, drawn up by an intelligent friend, who has given much
more attention to it than I have done.

1. The nerve of the limb of an animal being laid bare, and surrounded
with a piece of sheet lead, or of tinfoil, if a communication be formed
between the nerve thus armed and any of the neighbouring muscles, by
means of a piece of zinc, strong contractions will be produced in the
limb.

2. If a portion of the nerve which has been laid bare be armed as above,
contractions will be produced as powerfully, by forming the
communication between the armed and bare part of the nerve, as between
the armed part and muscle.

3. A similar effect is produced by arming a nerve and simply touching
the armed part of the nerve with the metallic conductor.

4. Contractions will take place if a muscle be armed, and a
communication be formed by means of the conductor between it and a
neighbouring nerve. The same effect will be produced if the
communication be formed between the armed muscle and another muscle,
which is contiguous to it.

5. Contractions may be produced in the limb of an animal by bringing the
pieces of metal into contact with each other at some distance from the
limb, provided the latter make part of a line of communication between
the two metallic conductors.

The experiment which proves this is made in the following manner. The
amputated limb of an animal being placed upon a table, let the operator
hold with one hand the principal nerve, previously laid bare, and in the
other let him hold a piece of zinc; let a small plate of lead or silver
be then laid upon the table, at some distance from the limb, and a
communication be formed, by means of water, between the limb and the
part of the table where the metal is lying. If the operator touch the
piece of silver with the zinc, contractions will be produced in the limb
the moment that the metals come into contact with each other. The same
effect will be produced if the two pieces of metal be previously placed
in contact, and the operator touch one of them with his finger. This
fact was discovered by Mr. William Cruikshank.

6. Contractions can be produced in the amputated leg of a frog, by
putting it into water, and bringing the two metals into contact with
each other at a small distance from the limb.

7. The influence which has passed through, and excited contractions in,
one limb, may be made to pass through, and excite contractions in,
another limb. In performing this experiment it is necessary to attend to
the following circumstances: let two amputated limbs of a frog be taken;
let one of them be laid upon a table, and its foot be folded in a piece
of silver; let a person lift up the nerve of this limb with a silver
probe, and another person hold in his hand a piece of zinc, with which
he is to touch the silver including the foot; let the person holding the
zinc in one hand catch with the other the nerve of the second limb, and
he who touches the nerve of the first limb is to hold in his other hand
the foot of the second; let the zinc now be applied to the silver
including the foot of the first limb, and contractions will immediately
be excited in both limbs.

8. The heart is the only involuntary muscle in which contractions can be
excited by these experiments.

9. Contractions are produced more strongly, the farther the coating is
placed from the origin of the nerve.

10. Animals which were almost dead have been found to be considerably
revived by exciting this influence.

11. When these experiments are repeated upon an animal that has been
killed by opium, or by the electric shock, very slight contractions are
produced; and no contractions whatever will take place in an animal that
has been killed by corrosive sublimate, or that has been starved to
death.

12. Zinc appears to be the best exciter when applied to gold, silver,
molybdena, steel, or copper. The latter metals, however, excite but
feeble contractions when applied to each other. Next to zinc, in contact
with these metals, tin and lead, and silver and lead, appear to be the
most powerful exciters.

At least two kinds of fishes, the _torpedo_ and the _electrical eel_,
have a voluntary power of giving so strong a shock to the water in which
they swim, as to affect fishes and other animals which come near them;
and by a conducing communication between different parts of these
fishes, an electric shock may be given exactly like that of the Leyden
phial, which will be described hereafter; and if the communication be
interrupted, a flash of electric light will be perceived.

The growth of vegetables is also quickened by electricity.




LECTURE XXXV.


_The same Subject continued._

No electric can be excited without producing electric appearances in the
body with which it is excited, provided that body be insulated; for this
insulated rubber will attract light bodies, give sparks, and make a
snapping noise, upon the approach of a conductor, as well as the excited
electric.

If an insulated conductor be pointed, or if a pointed conductor,
communicating with the earth, be held pretty near it, little or no
electric appearance will be exhibited, only a light will appear at each
of the points during the act of excitation, and a current of air will be
sensible from off them both.

The effect of pointed bodies is best explained on the supposition of the
electric matter in one body repelling that in another; and consequently
the electricity belonging to a body with a large surface making a
greater resistance to the entrance of foreign electricity than that
belonging to a smaller.

These two electricities, viz. that of the excited electric, and that of
the rubber, though similar to, are the reverse of, one another. A body
attracted by the one will be repelled by the other, and they will
attract, and in all respects act upon, one another more sensibly than
upon other bodies; so that two pieces of glass or silk possessed of
contrary electricities will cohere firmly together, and require a
considerable force to separate them.

These two electricities having been first discovered by producing one of
them from glass, and the other from amber, sealing-wax, sulphur, rosin,
&c. first obtained the names of _vitreous_ and _resinous_ electricity;
and it being afterwards imagined that one of them was a redundancy, and
the other a deficiency, of a supposed electric fluid, the former has
obtained the name of _positive_, and the latter that of _negative_,
electricity; and these terms are now principally in use.

Positive and negative electricity may be distinguished from each other
by the manner in which they appear at the points of bodies. From a
pointed body electrified positively, there issues a stream of light,
divided into denser streams, at the extremities; whereas, when the point
is electrified negatively, the light is more minutely divided, and
diffused equally. The former of these is called a _brush_, and the
latter a _star_.

If a conductor not insulated be brought within the atmosphere (that is
the sphere of action) of any electrified body, it acquires the
electricity opposite to that of the electrified body, and the nearer it
is brought, the stronger opposite electricity does it acquire, till the
one receive a spark from the other, and then the electricity of both
will be discharged.

The electric substance which separates the two conductors possessing
these two opposite kinds of electricity, is said to be _charged_.
Plates of glass are the most convenient for this purpose, and the
thinner the plate the greater is the charge it is capable of holding.
The conductors contiguous to each side of the glass are called their
_coating_.

Agreeably to the above-mentioned general principle, it is necessary that
one side of the charged glass have a communication with the rubber,
while the other receives the electricity from the conductor, or with the
conductor, while the other receives from the rubber.

It follows also, that the two sides of the plate thus charged are always
possessed of the two opposite electricities; that side which
communicates with the excited electric having the electricity of the
electric, and that which communicates with the rubber, that of the
rubber.

There is, consequently, a very eager attraction between these two
electricities with which the different sides of the plate are charged,
and when a proper communication is made by means of conductors, a flash
of electric light, attended with a report (which is greater or less in
proportion to the quantity of electricity communicated to them, and the
goodness of the conductors) is perceived between them, and the
electricity of both sides is thereby discharged.

The substance of the glass itself in, or upon, which these electricities
exist, is impervious to electricity, and does not permit them to unite;
but if they be very strong, and the plate of glass very thin, they will
force a passage through the glass. This, however, always breaks the
glass, and renders it incapable of another charge.

The flash of light, together with the explosion between the two opposite
sides of a charged electric, is generally called the _electric shock_,
on account of the disagreeable sensation it gives any animal whose body
is made use of to form the communication been them.

The electric shock is always found to perform the circuit from one side
of the charged glass to the other by the shortest passage through the
best conductors. Common communicated electricity also observes the same
rule in its transmission from one body to another.

It has not been found, that the electric shock takes up any sensible
space of time in being transmitted to the greatest distances.

The electric shock, as also the common electric spark, displaces the air
through which it passes; and if its passage from conductor to conductor
be interrupted by non-conductors of a moderate thickness, it will rend
and tear them in its passage, in such a manner as to exhibit the
appearance of a sudden expansion of the air about the center of the
shock.

If the electric circuit be interrupted, the electric matter, during the
discharge, will pass to any other body that lies near its path, and
instantly return. This may be called the _lateral explosion_. The effect
of this lateral explosion through a brass chain, when the quantity of
electricity is very great, will be the discolouring and partial burning
of the paper on which it lies.

If a great quantity of electricity be accumulated, as in a _battery_,
the explosion will pass over the surfaces of imperfect conductors
without entering them, and the effect will be a strong _concussion_ of
the substance. Also the electric matter thus accumulated and condensed
will, by its repulsion, form _concentric circles_, which will appear by
melting the surface of a flat piece of metal on which the explosion is
received.

If an electric shock, or strong spark, be made to pass through, or over,
the belly of a muscle, it forces it to contract, as in a convulsion.

If a strong shock be sent through a small animal body, it will often
deprive it instantly of life.

When the electric shock is very strong, it will give polarity to
magnetic needles, and sometimes it reverses their poles.

Great shocks, by which animals are killed, are said to hasten
putrefaction.

Electricity and lightning are in all respects the same thing; since
every effect of lightning may be imitated by electricity, and every
experiment in electricity may be made with lightning, brought down from
the clouds by means of insulated pointed rods of metal.




LECTURE XXXVI.


_The same Subject continued._

Three curious and important instruments, which are among the latest
improvements in electricity, deserve a particular explanation, and in
all of them the effect depends upon the general principles mentioned
above, viz. that bodies placed within the influence, or, as it is
usually termed, within the atmosphere, of an electrified body, are
affected by a contrary electricity, and that these two electricities
mutually attract each other. These instruments are the _electrophorus_,
the _condenser_ of electricity, and the _doubler_ of it.

The electrophorus consists of an insulated conducting plate applied to
an insulated electric. If the latter have any electricity communicated
to it, for example the negative, the positive electricity of the former
will be attracted by it, and consequently the plate will be capable of
receiving electricity from any body communicating with the earth; being,
in this situation, capable of containing more electricity than its
natural quantity. Consequently, when it is removed from the lower plate,
and the whole of its electricity equally diffused through it, it will
appear to have a redundance, and therefore will give a spark to any body
communicating with the earth. Being then replaced upon the electric, and
touched by any body communicating with the earth, it will be again
affected as before, and give a spark on being raised; and this process
may be continued at pleasure, the electrophorus supplying the place of
any other electrical machine.

If the conducting plate of the electrophorus be applied to a piece of
dry wood, marble, or any other substance through which electricity can
pass but very slowly, or if the insulated conducting plate be covered
with a piece of thin silk, which will make some resistance to the
passage of electricity, and it be then applied to another plate
communicating with the earth; and if, in either of these cases, a body
with a large surface possessed of a weak electricity be applied to the
conducting plate, the weak electricity not being able to overcome the
obstruction presented to it, so as to be communicated to the other
plate, will affect it with the contrary electricity, and this reacting
on the first plate, will condense its electricity on that part of the
plate to which it is contiguous; in consequence of which its capacity of
receiving electricity will be increased; so that on the separation of
the two plates, that electricity which was before condensed, being
equally diffused through the whole plate, will have a greater intensity
than it had before, attracting light bodies, or even giving a spark,
when the body from which it received its electricity was incapable of
it. For though it contained a great quantity of electricity, it was
diffused through so large a space that its intensity was very small.
This instrument is therefore called a _condenser of electricity_.

If an insulated plate of metal possessing the smallest degree of
electricity be presented very near to another plate communicating with
the earth, it will affect this plate with the opposite electricity; and
this being, in the same manner, applied to a third plate, will put it
into the same state with the first. If then these two plates be joined,
and the first plate be presented to either of them, its own electricity
being attracted by that of the plate presented, that of the other will
be drawn into it, so that its quantity will be doubled. The same process
being repeated, will again double the electricity of this plate, till,
from being quite insensible to the most exquisite electrometer, it will
become very conspicuous, or even give sparks. This instrument is
therefore called a _doubler of electricity_, of excellent use in
ascertaining the quality of atmospherical electricity when ever so
small. If this instrument be so constructed that these three plates can
be successively presented to one another by the revolution of one of
them on an axis, it is called the _revolving doubler_; and in this form
it is most convenient for use.


THE END.


    Transcriber's Note: Details of corrections

    |Position                            |Original     |Correction    |
    |                                    |             |              |
    |Lecture I, first paragraph          |limestone    |lime-stone    |
    |Lecture I, last paragraph           |_attraction_ |_attraction_, |
    |Lecture III, first paragraph        |viz          |viz.          |
    |Lecture III, last paragraph         |1.           |1             |
    |Lecture XVII, penultimate paragraph |dissoved     |dissolved     |
    |Lecture XIX, "Of Calcareous Earth"  |hundreth     |hundredth     |
    |Lecture XXVI, "Of Semi-metals"      |ignates      |ignites       |
    |Lecture XXVIII, first paragraph     |animony      |antimony      |





End of the Project Gutenberg EBook of Heads of Lectures on a Course of
Experimental Philosophy: Particularly Including Chemistry, by Joseph Priestley

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