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Alembic Club Reprints--No. 1.

EXPERIMENTS

UPON

MAGNESIA ALBA,

QUICKLIME,

AND SOME OTHER

ALCALINE SUBSTANCES.


BY JOSEPH BLACK, M.D.,
_Professor of Chemistry in the University of Edinburgh, 1766-1797_.

(1755.)


Edinburgh:
PUBLISHED BY THE ALEMBIC CLUB.

_Edinburgh Agent:_
WILLIAM F. CLAY, 18 TEVIOT PLACE.

_London Agents:_
SIMPKIN, MARSHALL, HAMILTON, KENT, & CO. LTD.

1898.

[Illustration: Insignia]




PREFACE.


Black's Paper entitled "Experiments upon Magnesia Alba, Quicklime, and
some other Alcaline Substances" was read in June 1755, and was first
published in "Essays and Observations, Physical and Literary. Read
before a Society in Edinburgh, and Published by them," Volume II.,
Edinburgh, 1756; pp. 157-225. It was subsequently reprinted several
times during the life of the author, not only in later editions of these
Essays, but also in a separate form. Copies of the original Paper are
now very difficult to obtain, and the later reprints have also become
scarce.

The present reprint is a faithful copy of the Paper as it first appeared
in 1756, the spelling, &c., of the original having been carefully
reproduced.

The Paper constitutes a highly important step in the laying of the
foundations of chemistry as an exact science, and furnishes a model of
carefully planned experimental investigation, and of clear reasoning
upon the results of experiment. It is neither so widely read by the
younger chemists nor is it so readily accessible as it ought to be, and
the object of the Alembic Club in issuing it as the first volume of a
series of Reprints of historically important contributions to Chemistry,
is to place it within easy reach of every student of Chemistry and of
the History of Chemistry.

The student's attention may be particularly called to Black's tacit
adoption of the quantitative method in a large number of his
experiments, and to the way in which he bases many of his conclusions
upon the results obtained in these experiments. Even yet it is very
frequently stated that the introduction of the quantitative method into
Chemistry (which did not by any means originate with Black) took place
at a considerably later date.

L. D.




EXPERIMENTS

UPON

MAGNESIA ALBA, QUICKLIME,

AND SOME OTHER

ALCALINE SUBSTANCES;

BY JOSEPH BLACK, M.D.[1]

PART I.


Hoffman, in one of his observations, gives the history of a powder
called _magnesia alba_, which had long been used and esteemed as a mild
and tasteless purgative; but the method of preparing it was not
generally known before he made it public.[2]

It was originally obtained from a liquor called the _mother of nitre_,
which is produced in the following manner:

Salt-petre is separated from the brine which first affords it, or from
the water with which it is washed out of nitrous earths, by the process
commonly used in crystallizing salts. In this process the brine is
gradually diminished, and at length reduced to a small quantity of an
unctuous bitter saline liquor, affording no more salt-petre by
evaporation; but, if urged with a brisk fire, drying up into a confused
mass which attracts water strongly, and becomes fluid again when exposed
to the open air.

To this liquor the workmen have given the name of the _mother of
nitre_; and _Hoffman_, finding it composed of the _magnesia_ united to
an acid, obtained a separation of these, either by exposing the compound
to a strong fire in which the acid was dissipated and the _magnesia_
remained behind, or by the addition of an alkali which attracted the
acid to itself: and this last method he recommends as the best. He
likewise makes an inquiry into the nature and virtues of the powder thus
prepared; and observes, that it is an absorbent earth which joins
readily with all acids, and must necessarily destroy any acidity it
meets in the stomach; but that its purgative power is uncertain, for
sometimes it has not the least effect of that kind. As it is a mere
insipid earth, he rationally concludes it to be purgative only when
converted into a sort of neutral salt by an acid in the stomach, and
that its effect is therefore proportional to the quantity of this acid.

Altho' _magnesia_ appears from this history of it to be a very innocent
medicine, yet having observed, that some hypochondriacs who used it
frequently, were subject to flatulencies and spasms, he seems to have
suspected it of some noxious quality. The circumstances however which
gave rise to his suspicion, may very possibly have proceeded from the
imprudence of his patients, who, trusting too much to _magnesia_, (which
is properly a palliative in that disease,) and neglecting the assistance
of other remedies, allowed their disorder to increase upon them. It may
indeed be alledged, that _magnesia_, as a purgative, is not the most
eligible medicine for such constitutions, as they agree best with those
that strengthen, stimulate and warm; which the saline purges commonly
used are not observed to do. But there seems at least to be no objection
to its use when children are troubled with an acid in their stomach; for
gentle purging in this case is very proper, and it is often more
conveniently procured by means of _magnesia_ than of any other medicine,
on account of its being intirely insipid.

The above-mentioned Author observing, some time after, that a bitter
saline liquor, similar to that obtained from the brine of salt-petre,
was likewise produced by the evaporation of those waters which contain
common salt, had the curiosity to try if this would also yield a
_magnesia_. The experiment succeeded: and he thus found out another
process for obtaining this powder, and at the same time assured himself
by experiments, that the product from both was exactly the same.[3]

My curiosity led me some time ago to inquire more particularly into the
nature of _magnesia_, and especially to compare its properties with
those of the other absorbent earths, of which there plainly appeared to
me to be very different kinds, altho' commonly confounded together under
one name. I was indeed led to this examination of the absorbent earths,
partly by the hope of discovering a new sort of lime and lime-water,
which might possibly be a more powerful solvent of the stone than that
commonly used; but was disappointed in my expectations.

I have had no opportunity of seeing _Hoffman's_ first _magnesia_ or the
liquor from which it is prepared, and have therefore been obliged to
make my experiments upon the second.

In order to prepare it, I at first employed the bitter saline liquor
called _bittern_, which remains in the pans after the evaporation of sea
water. But as that liquor is not always easily procured, I afterwards
made use of a salt called _epsom-salt_, which is separated from the
bittern by crystallization, and is evidently composed of _magnesia_ and
the vitriolic acid.

There is likewise a spurious kind of Glauber salt, which yields plenty
of _magnesia_, and seems to be no other than the epsom salt of sea water
reduced to crystals of a larger size. And common salt also affords a
small quantity of this powder; because being separated from the bittern
by one hasty crystallization only, it necessarily contains a portion of
that liquor.

Those who would prepare a _magnesia_ from epsom-salt, may use the
following process.

Dissolve equal quantities of epsom-salt, and of pearl ashes separately
in a sufficient quantity of water; purify each solution from its dregs,
and mix them accurately together by violent agitation: then make them
just to boil over a brisk fire.

Add now to the mixture three or four times its quantity of hot water;
after a little agitation, allow the _magnesia_ to settle to the bottom,
and decant off as much of the water as possible. Pour on the same
quantity of cold water; and, after settling, decant it off in the same
manner. Repeat this washing with the cold water ten or twelve times: or
even oftner, if the _magnesia_ be required perfectly pure for chemical
experiments.

When it is sufficiently washed, the water may be strained and squeezed
from it in a linen cloth; for very little of the _magnesia_ passes
thro'.

The alkali in the mixture uniting with the acid, separates it from the
_magnesia_; which not being of itself soluble in water, must
consequently appear immediately under a solid form. But the powder which
thus appears is not intirely _magnesia_; part of it is the neutral salt,
formed from the union of the acid and alkali. This neutral salt is
found, upon examination, to agree in all respects with vitriolated
tartar, and requires a large quantity of hot water to dissolve it. As
much of it is therefore dissolved as the water can take up; the rest is
dispersed thro' the mixture in the form of a powder. Hence the necessity
of washing the _magnesia_ with so much trouble; for the first affusion
of hot water is intended to dissolve the whole of the salt, and the
subsequent additions of cold water to wash away this solution.

The caution given of boiling the mixture is not unnecessary; if it be
neglected, the whole of the _magnesia_ is not accurately separated at
once; and by allowing it to rest for some time, that powder concretes
into minute grains, which, when viewed with the microscope, appear to be
assemblages of needles diverging from a point. This happens more
especially when the solutions of the epsom-salt and of the alkali are
diluted with too much water before they are mixed together. Thus, if a
dram of epsom-salt and of salt of tartar be dissolved each in four
ounces of water, and be mixed, and then allowed to rest three or four
days, the whole of the _magnesia_ will be formed into these grains. Or
if we filtrate the mixture soon after it is made, and heat the clear
liquor which passes thro'; it will become turbid, and deposite a
_magnesia_.

I had the curiosity to satisfy myself of the purgative power of
_magnesia_, and of _Hoffman's_ opinion concerning it, by the following
easy experiment. I made a neutral salt of _magnesia_ and distilled
vinegar; choosing this acid as being, like that in weak stomachs, the
product of fermentation. Six drams of this I dissolved in water, and
gave to a middle-aged man, desiring him to take it by degrees. After
having taken about a third, he desisted, and purged four times in an
easy and gentle manner. A woman of a strong constitution got the
remainder as a brisk purgative, and it operated ten times without
causing any uneasiness. The taste of this salt is not disagreeable, and
it appears to be rather of the cooling than of the acrid kind.

Having thus given a short sketch of the history and medical virtues of
_magnesia_, I now proceed to an account of its chemical properties. By
my first experiments, I intended to learn what sort of neutral salts
might be obtained by joining it to each of the vulgar acids; and the
result was as follows.

Magnesia is quickly dissolved with violent effervescence, or explosion
of air, by the acids of vitriol, nitre, and of common salt, and by
distilled vinegar; the neutral saline liquors thence produced having
each their peculiar properties.

That which is made with the vitriolic acid, may be condensed into
crystals similar in all respects to epsom-salt.

That which is made with the nitrous is of a yellow colour, and yields
saline crystals, which retain their form in a very dry air, but melt in
a moist one.

That which is produced by means of spirit of salt, yields no crystals;
and if evaporated to dryness, soon melts again when exposed to the air.

That which is obtained from the union of distilled vinegar with
_magnesia_, affords no crystals by evaporation, but is condensed into a
saline mass, which, while warm, is extremely tough and viscid, very much
resembling a strong glue both in colour and consistence, and becomes
brittle when cold.

By these experiments _magnesia_ appears to be a substance very different
from those of the calcarious class; under which I would be understood to
comprehend all those that are converted into a perfect quick-lime in a
strong fire, such as _lime-stone_, _marble_, _chalk_, those _spars_ and
_marles_ which effervesce with aqua fortis, all _animal shells_ and the
bodies called _lithophyta_. All of these, by being joined with acids,
yield a set of compounds which are very different from those we have
just now described. Thus, if a small quantity of any calcarious matter
be reduced to a fine powder and thrown into spirit of vitriol, it is
attacked by this acid with a brisk effervescence; but little or no
dissolution ensues. It absorbs the acid, and remains united with it in
the form of a white powder, at the bottom of the vessel, while the
liquor has hardly any taste, and shews only a very light cloud upon the
addition of alkali.[4]

The same white powder is also formed when spirit of vitriol is added to
a calcarious earth dissolved in any other acid; the vitriolic expelling
the other acid, and joining itself to the earth by a stronger
attraction; and upon this account the _magnesia_ of sea-water seems to
be different from either of those described by _Hoffman_. He says
expressly, that the solutions of each of his powders, or, what is
equivalent, that the liquors from which they are obtained, formed a
coagulum, and deposited a white powder, when he added the vitriolic
acid;[5] which experiment I have often tried with the marine bittern,
but without success. The coagulum thus formed in the mother of nitre may
be owing to a quantity of quick-lime contained in it; for quick-lime is
used in extracting the salt-petre from its matrix. But it is more
difficult to account for the difference between _Hoffman's_ bittern and
ours, unless we will be satisfied to refer it to this, that he got his
from the waters of salt springs, which may possibly be different from
those of the sea.

Magnesia is not less remarkably distinguished from the calcarious
earths, by joining it to the nitrous and vegetable acids, than to the
vitriolic. Those earths, when combined with spirit of nitre, cannot be
reduced to a crystalline form, and if they are dissolved in distilled
vinegar, the mixture spontaneously dries up into a friable salt.

Having thus found _magnesia_ to differ from the common alkaline earths,
the object of my next inquiry was its peculiar degree of attraction for
acids, or what was the place due to it in Mr. _Geoffroy's_ table of
elective attractions.

Three drams of _magnesia_ in fine powder, an ounce of salt ammoniac, and
six ounces of water were mixed together, and digested six days in a
retort joined to a receiver.

During the whole time, the neck of the retort was pointed a little
upwards, and the most watery part of the vapour, which was condensed
there, fell back into its body. In the beginning of the experiment, a
volatile salt was therefore collected in a dry form in the receiver, and
afterwards dissolved into spirit.

When all was cool, I found in the retort a saline liquor, some
undissolved _magnesia_, and some salt ammoniac crystallized. The saline
liquor was separated from the other two, and then mixed with the
alkaline spirit. A coagulum was immediately formed, and a _magnesia_
precipitated from the mixture.

The _magnesia_ which had remained in the retort, when well washed and
dried, weighed two scruples and fifteen grains.

We learn by the latter part of this experiment, that the attraction of
the volatile alkali for acids is stronger than that of _magnesia_, since
it separated this powder from the acid to which it was joined. But it
also appears, that a gentle heat is capable of overcoming this
superiority of attraction, and of gradually elevating the alkali, while
it leaves the less volatile acid with the _magnesia_.

Dissolve a dram of any calcarious substance in the acid of nitre or of
common salt, taking care that the solution be rendered perfectly
neutral, or that no superfluous acid be added. Mix with this solution a
dram of _magnesia_ in fine powder, and digest it in the heat of boiling
water about twenty four hours; then dilute the mixture with double its
quantity of water, and filtrate. The greatest part of the earth now left
in the filtre is calcarious, and the liquor which passed thro', if mixed
with a dissolved alkali, yields a white powder, the largest portion of
which is a true _magnesia_.

From this experiment it appears, that an acid quits a calcarious earth
to join itself to _magnesia_; but the exchange being performed slowly,
some of the _magnesia_ is still undissolved, and part of the calcarious
earth remains yet joined to the acid.

When a small quantity of _magnesia_ is thrown into a solution of the
corrosive sublimate of mercury, it soon separates part of the mercury in
the form of a dark red powder, and is itself dissolved.

Imagining that I perceived some resemblance between the properties of
_magnesia_ and those of alkalis, I was led to try what change this
substance would suffer from the addition of quick-lime, which alters in
such a peculiar manner the alkaline salts.

Twenty seven grains of _magnesia_ in fine powder were mixed with
eighteen ounces of lime-water in a flask, which was corked close and
shaken frequently for four days. During this time, I frequently dipp'd
into it little bits of paper, which were coloured with the juice of
violets; and these became green as soon as they touched the water, until
the fourth day, when their colour did not seem to be altered. The water
being now poured off, was intirely insipid, and agreed in every
chemical trial with pure water. The powder, after being perfectly well
dried, weighed thirty seven grains. It did not dissolve intirely in
spirit of vitriol; but, after a brisk effervescence, part of it subsided
in the same manner as the calcarious earths, when mixed with this acid.

When I first tried this experiment, I was at the trouble of digesting
the mixture in the heat of boiling water, and did not then know that it
would succeed in the heat of the air. But Dr. _Alston_, who has obliged
the world with many curious and useful discoveries on the subject of
quick-lime, having had occasion to repeat it, I learned from him that
heat is not necessary; and he has moreover added an useful purpose to
which this property of _magnesia_ may be applied; I mean the sweetening
of water at sea, with which lime may have been mixed to prevent its
putrefaction.

That part of the dried powder which does not dissolve in spirit of
vitriol, consists of the lime separated from the water.

Quick-lime itself is also rendered mild by _magnesia_, if these two are
well rubbed together and infused with a small quantity of water.

By the following experiments, I proposed to know whether this substance
could be reduced to a quick-lime.

An ounce of _magnesia_ was exposed in a crucible for about an hour to
such a heat as is sufficient to melt copper. When taken out, it weighed
three drams and one scruple, or had lost 7/12 of its former weight.

I repeated, with the _magnesia_ prepared in this manner, most of those
experiments I had already made upon it before calcination, and the
result was as follows.

It dissolves in all the acids, and with these composes salts exactly
similar to those described in the first set of experiments: but what is
particularly to be remarked, it is dissolved without any the least
degree of effervescence.

It slowly precipitates the corrosive sublimate of mercury in the form of
a black powder.

It separates the volatile alkali in salt ammoniac from the acid, when it
is mixed with a warm solution of that salt. But it does not separate an
acid from a calcarious earth, nor does it induce the least change upon
lime-water.

Lastly, when a dram of it is digested with an ounce of water in a bottle
for some hours, it does not make any the least change in the water. The
_magnesia_, when dried, is found to have gained ten grains; but it
neither effervesces with acids, nor does it sensibly affect lime-water.

Observing _magnesia_ to lose such a remarkable proportion of its weight
in the fire, my next attempts were directed to the investigation of this
volatile part, and, among other experiments, the following seemed to
throw some light upon it.

Three ounces of _magnesia_ were distilled in a glass retort and
receiver, the fire being gradually increased until the _magnesia_ was
obscurely red hot. When all was cool, I found only five drams of a
whitish water in the receiver, which had a faint smell of the spirit of
hartshorn, gave a green colour to the juice of violets, and rendered the
solutions of corrosive sublimate and of silver very slightly turbid. But
it did not sensibly effervesce with acids.

The _magnesia_, when taken out of the retort, weighed an ounce, three
drams, and thirty grains, or had lost more than the half of its weight.
It still effervesced pretty briskly with acids, tho' not so strongly as
before this operation.

The fire should have been raised here to the degree requisite for the
perfect calcination of _magnesia_. But even from this imperfect
experiment, it is evident, that of the volatile parts contained in that
powder, a small proportion only is water; the rest cannot, it seems, be
retained in vessels, under a visible form. Chemists have often observed,
in their distillations, that part of a body has vanished from their
senses, notwithstanding the utmost care to retain it; and they have
always found, upon further inquiry, that subtile part to be air, which
having been imprisoned in the body, under a solid form, was set free and
rendered fluid and elastic by the fire. We may therefore safely
conclude, that the volatile matter, lost in the calcination of
_magnesia_, is mostly air; and hence the calcined _magnesia_ does not
emit air, or make an effervescence, when mixed with acids.

The water, from its properties, seems to contain a small portion of
volatile alkali, which was probably formed from the earth, air, and
water, or from some of these combined together; and perhaps also from a
small quantity of inflammable matter which adhered accidentally to the
_magnesia_. Whenever Chemists meet with this salt, they are inclined to
ascribe its origin to some animal, or putrid vegetable, substance; and
this they have always done, when they obtained it from the calcarious
earths, all of which afford a small quantity of it. There is, however,
no doubt that it can sometimes be produced independently of any such
mixture, since many fresh vegetables and tartar afford a considerable
quantity of it. And how can it, in the present instance, be supposed,
that any animal or vegetable matter adhered to the _magnesia_, while it
was dissolved by an acid, separated from this by an alkali, and washed
with so much water?

Two drams of _magnesia_ were calcined in a crucible, in the manner
described above, and thus reduced to two scruples and twelve grains.
This calcined _magnesia_ was dissolved in a sufficient quantity of
spirit of vitriol, and then again separated from the acid by the
addition of an alkali, of which a large quantity is necessary for this
purpose. The _magnesia_ being very well washed and dryed, weighed one
dram and fifty grains. It effervesced violently, or emitted a large
quantity of air, when thrown into acids, formed a red powder when mixed
with a solution of sublimate, separated the calcarious earths from an
acid, and sweetened lime-water: and had thus recovered all those
properties which it had but just now lost by calcination: nor had it
only recovered its original properties, but acquired besides an addition
of weight nearly equal to what had been lost in the fire; and, as it is
found to effervesce with acids, part of the addition must certainly be
air.

This air seems to have been furnished by the alkali from which it was
separated by the acid; for Dr. _Hales_ has clearly proved, that alkaline
salts contain a large quantity of fixed air, which they emit in great
abundance when joined to a pure acid. In the present case, the alkali is
really joined to an acid, but without any visible emission of air; and
yet the air is not retained in it: for the neutral salt, into which it
is converted, is the same in quantity, and in every other respect, as if
the acid employed had not been previously saturated with _magnesia_, but
offered to the alkali in its pure state, and had driven the air out of
it in their conflict. It seems therefore evident, that the air was
forced from the alkali by the acid, and lodged itself in the _magnesia_.

These considerations led me to try a few experiments, whereby I might
know what quantity of air is expelled from an alkali, or from
_magnesia_, by acids.

Two drams of a pure fixed alkaline salt, and an ounce of water, were put
into a Florentine flask, which, together with its contents, weighed two
ounces and two drams. Some oil of vitriol diluted with water was dropt
in, until the salt was exactly saturated; which it was found to be, when
two drams, two scruples, and three grains of this acid had been added.
The vial with its contents now weighed two ounces, four drams, and
fifteen grains. One scruple, therefore, and eight grains were lost
during the ebullition, of which a trifling portion may be water, or
something of the same kind. The rest is air.

The celebrated _Homberg_ has attempted to estimate the quantity of solid
salt contained in a determined portion of the several acids. He
saturated equal quantities of an alkali with each of them; and,
observing the weight which the alkali had gained, after being perfectly
dryed, took this for the quantity of solid salt contained in that share
of the acid which performed the saturation. But we learn from the above
experiment, that his estimate was not accurate, because the alkali loses
weight as well as gains it.

Two drams of _magnesia_, treated exactly as the alkali in the last
experiment, were just dissolved by four drams, one scruple, and seven
grains of the same acid liquor, and lost one scruple and sixteen grains
by the ebullition.

Two drams of _magnesia_ were reduced, by the action of a violent fire,
to two scruples and twelve grains, with which the same process was
repeated, as in the two last experiments; four drams, one scruple, and
two grains of the same acid were required to compleat the solution, and
no weight was lost in the experiment.

As in the separation of the volatile from the fixed parts of bodies, by
means of heat, a small quantity of the latter is generally raised with
the former; so the air and water, originally contained in the
_magnesia_, and afterwards dissipated by the fire, seem to have carried
off a small part of the fixed earth of this substance. This is probably
the reason, why calcined _magnesia_ is saturated with a quantity of
acid, somewhat less than what is required to dissolve it before
calcination: and the same may be assigned as one cause which hinders us
from restoring the whole of its original weight, by solution and
precipitation.

I took care to dilute the vitriolic acid, in order to avoid the heat and
ebullition which it would otherwise have excited in the water; and I
chose a Florentine flask, on account of its lightness, capacity, and
shape, which is peculiarly adapted to the experiment; for the vapours
raised by the ebullition circulated for a short time, thro' the wide
cavity of the vial, but were soon collected upon its sides, like dew,
and none of them seemed to reach the neck, which continued perfectly dry
to the end of the experiment.

We now perceive the reason, why crude and calcined _magnesia_, which
differ in many respects from one another, agree however in composing the
same kind of salt, when dissolved in any particular acid; for the crude
_magnesia_ seems to differ from the calcined chiefly by containing a
considerable quantity of air, which air is unavoidably dissipated and
lost during the dissolution.

From our experiments, it seems probable, that the increase of weight
which some metals acquire, by being first dissolved in acids, and then
separated from them again by alkalis, proceeds from air furnished by the
alkalis. And that in the _aurum fulminans_, which is prepared by the
same means, this air adheres to the gold in such a peculiar manner,
that, in a moderate degree of heat, the whole of it recovers its
elasticity in the same instant of time; and thus, by the violent shock
which it gives to the air around, produces the loud crack or fulmination
of this powder. Those who will imagine the explosion of such a minute
portion of fixed air, as can reside in the _aurum fulminans_, to be
insufficient for the excessive loudness of the noise, will consider,
that it is not a large quantity of motion communicated to the air, but
rather a smart stroke which produces sound, and that the explosion of
but a few particles of fixed air may be capable of causing a loud noise,
provided they all recover their spring suddenly, and in the same
instant.

The above experiments lead us also to conclude, that volatile alkalis,
and the common absorbent earths, which lose their air by being joined to
acids, but shew evident signs of their having recovered it, when
separated from them by alkalis, received it from these alkalis which
lost it in the instant of their joining with the acid.

The following are a few experiments upon three of the absorbent earths,
made in order to compare them with one another, and with _magnesia_.

Suspecting that _magnesia_ might possibly be no other than a common
calcarious earth, which had changed its nature, by having been
previously combined with an acid, I saturated a small quantity of chalk
with the muriatic acid, separated the acid from it again by means of a
fixed alkali, and carefully washed away the whole of the salt.

The chalk when dryed was not found to have suffered any alteration; for
it effervesced with the vitriolic acid, but did not dissolve in it; and
when exposed to a violent fire, was converted into a quick-lime, in all
respects similar to that obtained from common chalk.

In another experiment of the same kind, I used the vitriolic acid with
the same event.

Any calcarious matter reduced to a fine powder, and thrown into a warm
solution of alum, immediately raises a brisk effervescence. But the
powder is not dissolved; it is rather increased in bulk: and if the
addition be repeated until it is no longer accompanied with
effervescence, the liquor loses all taste of the alum, and yields only a
very light cloud upon the admixture of an alkali.

From this experiment we learn, that acids attract the calcarious earths
more strongly than they do the earth of alum; and as the acid in this
salt is exactly the same with the vitriolic, it composes with the
calcarious earth a neutral substance, which is very difficultly soluble
in water, and therefore falls down to the bottom of the vessel along
with the earth of alum which is deprived of its acid. The light cloud
formed by the alkali proceeds from the minute portion of the calcarious
compound which saturates the water.

The earth of animal bones, when reduced to a fine powder and thrown into
a diluted vitriolic acid, gradually absorbs the acid in the same manner
as the calcarious earths, but without any remarkable effervescence. When
it is added to the nitrous or to the muriatic acid, it is slowly
dissolved. The compound liquor thence produced is extremely acrid, and
still changes the colour of the juice of violets to a red, even after it
is fully saturated with the absorbent. Distilled vinegar has little or
no effect upon this earth; for after a long digestion it still retains
its sour taste, and gives only a light cloud upon the addition of an
alkali.

By dropping a dissolved fixed alkali into a warm solution of alum, I
obtained the earth of this salt, which, after being well washed and
dried, was found to have the following properties.

It is dissolved in every acid but very slowly, unless assisted by heat.
The several solutions, when thoroughly saturated, are all astringent
with a slight degree of an acid taste, and they also agree with a
solution of alum in this, that they give a red colour to the infusion of
turnsol.

Neither this earth, nor that of animal bones, can be converted into
quick-lime by the strongest fire, nor do they suffer any change worth
notice. Both of them seem to attract acids but weakly, and to alter
their properties less when united to them than the other absorbents.




PART II.


In reflecting afterwards upon these experiments, an explication of the
nature of lime offered itself, which seemed to account, in an easy
manner, for most of the properties of that substance.

It is sufficiently clear, that the calcarious earths in their native
state, and that the alkalis and magnesia in their ordinary condition,
contain a large quantity of fixed air, and this air certainly adheres to
them with considerable force, since a strong fire is necessary to
separate it from magnesia, and the strongest is not sufficient to expel
it entirely from fixed alkalis, or take away their power of effervescing
with acid salts.

These considerations led me to conclude, that the relations between
fixed air and alkaline substances was somewhat similar to the relation
between these and acids; that as the calcarious earths and alkalis
attract acids strongly and can be saturated with them, so they also
attract fixed air, and are in their ordinary state saturated with it:
and when we mix an acid with an alkali or with an absorbent earth, that
the air is then set at liberty, and breaks out with violence; because
the alkaline body attracts it more weakly than it does the acid, and
because the acid and air cannot both be joined to the same body at the
same time.

I also imagined, that, when the calcarious earths are exposed to the
action of a violent fire, and are thereby converted into quick-lime,
they suffer no other change in their composition than the loss of a
small quantity of water and of their fixed air. The remarkable acrimony
which we perceive in them after this process, was not supposed to
proceed from any additional matter received in the fire, but seemed to
be an essential property of the pure earth, depending on an attraction
for those several substances which it then became capable of corroding
or dissolving, which attraction had been insensible as long as the air
adhered to the earth, but discovered itself upon the separation.

This supposition was founded upon an observation of the most frequent
consequences of combining bodies in chemistry. Commonly when we join two
bodies together, their acrimony or attraction for other substances
becomes immediately either less perceivable or entirely insensible;
altho' it was sufficiently strong and remarkable before their union, and
may be rendered evident again by disjoining them. A neutral salt, which
is composed of an acid and alkali, does not possess the acrimony of
either of its constituent parts. It can easily be separated from water,
has little or no effect upon metals, is incapable of being joined to
inflammable bodies, and of corroding and dissolving animals and
vegetables; so that the attraction both of the acid and alkali for these
several substances seems to be suspended till they are again separated
from one another.

Crude lime was therefore considered as a peculiar acrid earth rendered
mild by its union with fixed air: and quick-lime as the same earth, in
which, by having separated the air, we discover that acrimony or
attraction for water, for animal, vegetable, and for inflammable
substances.

That the calcarious earths really lose a large quantity of air when they
are burnt to quick-lime, seems sufficiently proved by an experiment of
Mr. _Margraaf_,[6] an exceedingly accurate and judicious Chemist. He
subjected eight ounces of _osteocolla_ to distillation in an earthen
retort, finishing his process with the most violent fire of a
reverberatory, and caught in the receiver only two drams of water, which
by its smell and properties shewed itself to be slightly alkaline. He
does not tell us the weight of the _osteocolla_ remaining in the retort,
and only says, that it was converted into quick-lime; but as no
calcarious earth can be converted into quick-lime, or bear the heat
which he applied without losing above a third of its weight, we may
safely conclude, that the loss in his experiment was proportional, and
proceeded chiefly from the dissipation of fixed air.

According to our theory, the relation of the calcarious earth to air and
water appeared to agree with the relation of the same earth to the
vitriolic and vegetable acids. As chalk for instance has a stronger
attraction for the vitriolic than for the vegetable acid, and is
dissolved with more difficulty when combined with the first, than when
joined to the second: so it also attracts air more strongly than water,
and is dissolved with more difficulty when saturated with air than when
compounded with water only.

A calcarious earth deprived of its air, or in the state of quick-lime,
greedily absorbs a considerable quantity of water, becomes soluble in
that fluid, and is then said to be slaked; but as soon as it meets with
fixed air, it is supposed to quit the water and join itself to the air,
for which it has a superior attraction, and is therefore restored to its
first state of mildness and insolubility in water.

When slaked lime is mixed with water, the fixed air in the water is
attracted by the lime, and saturates a small portion of it, which then
becomes again incapable of dissolution, but part of the remaining slaked
lime is dissolved and composes lime-water.

If this fluid be exposed to the open air, the particles of quick-lime
which are nearest the surface gradually attract the particles of fixed
air which float in the atmosphere. But at the same time that a particle
of lime is thus saturated with air, it is also restored to its native
state of mildness and insolubility; and as the whole of this change
must happen at the surface, the whole of the lime is successively
collected there under its original form of an insipid calcarious earth,
called the cream or crusts of lime-water.

When quick-lime itself is exposed to the open air, it absorbs the
particles of water and of fixed air which come within its sphere of
attraction, as it meets with the first of these in greatest plenty, the
greatest part of it assumes the form of slaked lime; the rest is
restored to its original state; and if it be exposed for a sufficient
length of time, the whole of it is gradually saturated with air, to
which the water as gradually yields its place.

We have already shown by experiment, that magnesia alba is a compound of
a peculiar earth and fixed air. When this substance is mixed with
lime-water, the lime shews a stronger attraction for fixed air than that
of the earth of magnesia; the air leaves this powder to join itself to
the lime. And as neither the lime when saturated with air, nor the
magnesia when deprived of it, are soluble in water, the lime-water
becomes perfectly pure and insipid, the lime which it contained being
mixed with the magnesia. But if the magnesia be deprived of air by
calcination before it is mixed with the lime-water, this fluid suffers
no alteration.

If quick-lime be mixed with a dissolved alkali, it likeways shews an
attraction for fixed air superior to that of the alkali. It robs this
salt of its air, and thereby becomes mild itself, while the alkali is
consequently rendered more corrosive, or discovers its natural degree of
acrimony or strong attraction for water, and for bodies of the
inflammable, and of the animal and vegetable kind; which attraction was
less perceivable as long as it was saturated with air. And the volatile
alkali when deprived of its air, besides this attraction for various
bodies, discovers likeways its natural degree of volatility, which was
formerly somewhat repressed by the air adhering to it, in the same
manner as it is repressed by the addition of an acid.

This account of lime and alkalis recommended itself by its simplicity,
and by affording an easy solution of many _phaenomena_, but appeared upon
a nearer view to be attended with consequences that were so very new and
extraordinary, as to render suspicious the principles from which they
were drawn.

I resolved however to examine, in a particular manner, such of these
consequences as were the most unavoidable, and found the greatest number
of them might be reduced to the following propositions:

     I. If we only separate a quantity of air from lime and alkalis,
     when we render them caustic they will be found to lose part of
     their weight in the operation, but will saturate the same quantity
     of acid as before, and the saturation will be performed without
     effervescence.

     II. If quick-lime be no other than a calcarious earth deprived of
     its air, and whose attraction for fixed air is stronger than that
     of alkalis, it follows, that, by adding to it a sufficient quantity
     of alkali saturated with air, the lime will recover the whole of
     its air, and be entirely restored to its original weight and
     condition: and it also follows, that the earth separated from
     lime-water by an alkali, is the lime which was dissolved in the
     water now restored to its original mild and insoluble state.

     III. If it be supposed that slaked lime does not contain any parts
     which are more firey, active or subtile than others, and by which
     chiefly it communicates its virtues to water; but that it is an
     uniform compound of lime and water: it follows, that, as part of it
     can be dissolved in water, the whole of it is also capable of being
     dissolved.

     IV. If the acrimony of the caustic alkali does not depend on any
     part of the lime adhering to it, a caustic or soap-ley will
     consequently be found to contain no lime, unless the quantity of
     lime employed in making it were greater than what is just
     sufficient to extract the whole air of the alkali; for then as much
     of the superfluous quick-lime might possibly be dissolved by the
     ley as would be dissolved by pure water, or the ley would contain
     as much lime as lime-water does.

     V. We have shewn in the former experiments, that absorbent earths
     lose their air when they are joined to an acid; but recover it, if
     separated again from that acid, by means of an ordinary alkali: the
     air passing from the alkali to the earth, at the same time that the
     acid passes from the earth to the alkali.

If the caustic alkali therefore be destitute of air, it will separate
magnesia from an acid under the form of a magnesia free of air, or which
will not effervesce with acids; and the same caustic alkali will also
separate a calcarious earth from acids under the form of a calcarious
earth destitute of air, but saturated with water, or under the form of
slaked lime.

These were all necessary conclusions from the above suppositions. Many
of them appeared too improbable to deserve any further attention: some
however, I found upon reflection, were already seconded by experience.
Thus _Hoffman_ has observed, that quick-lime does not effervesce with
spirit of vitriol;[7] and it is well known that the caustic spirit of
urine, or of salt ammoniac, does not emit air, when mixed with acids.
This consideration excited my curiosity, and determined me to inquire
into the truth of them all by way of experiment. I therefore engaged
myself in a set of trials; the history of which is here subjoined. Some
new facts are likeways occasionally mentioned; and here it will be
proper to inform the reader, that I have never mentioned any without
satisfying myself of their truth by experiment, tho' I have sometimes
taken the liberty to neglect describing the experiments when they seemed
sufficiently obvious.

Desiring to know how much of an acid a calcarious earth will absorb, and
what quantity of air is expelled during the dissolution, I saturated two
drams of chalk with diluted spirit of salt, and used the Florentine
flask, as related in a similar experiment upon magnesia. Seven drams and
one grain of the acid finished the dissolution, and the chalk lost two
scruples and eight grains of air.

This experiment was necessary before the following, by which I proposed
to inquire into the truth of the first proposition so far as it relates
to quick-lime.

Two drams of chalk were converted into a perfect quick-lime, and lost
two scruples and twelve grains in the fire. This quick-lime was slaked
or reduced to a milky liquor with an ounce of water, and then dissolved
in the same manner, and with the same acid, as the two drams of chalk in
the preceding experiment. Six drams, two scruples and fourteen grains of
the acid finished the saturation without any sensible effervescence or
loss of weight.

It therefore appears from these experiments, that no air is separated
from quick-lime by an acid, and that chalk saturates nearly the same
quantity of acid after it is converted into quick-lime as before.

With respect to the second proposition, I tried the following
experiments.

A piece of perfect quick-lime made from two drams of chalk, and which
weighed one dram and eight grains, was reduced to a very fine powder,
and thrown into a filtrated mixture of an ounce of a fixed alkaline salt
and two ounces of water. After a slight digestion, the powder being well
washed and dried, weighed one dram and fifty eight grains. It was
similar in every trial to a fine powder of ordinary chalk, and was
therefore saturated with air which must have been furnished by the
alkali.

A dram of pure salt of tartar was dissolved in fourteen pounds of
lime-water, and the powder thereby precipitated, being carefully
collected and dried, weighed one and fifty grains. When exposed to a
violent fire, it was converted into a true quick-lime, and had every
other quality of a calcarious earth.

This experiment was repeated with the volatile alkali, and also with the
fossil or alkali of sea-salt, and exactly with the same event.

The third proposition had less appearance of probability than the
foregoing; but, as an accurate experiment was the only test of its
truth, I reduced eight grains of perfect quick-lime made of chalk, to an
exceedingly subtile powder, by slaking it in two drams of distilled
water boiling hot, and immediately threw the mixture into eighteen
ounces of distilled water in a flask. After shaking it, a light
sediment, which floated thro' the liquor, was allowed to subside and
this, when collected with the greatest care, and dryed, weighed, as
nearly as I could guess, one third of a grain. The water tasted strongly
of the lime, had all the qualities of lime-water, and yielded twelve
grains of precipitate, upon the addition of salt of tartar. In repeating
this experiment, the quantity of sediment was sometimes less than the
above, and sometimes amounted to half a grain. It consisted partly of an
earth which effervesced violently with _aqua fortis_, and partly of an
ochry powder, which would not dissolve in that acid. The ochry powder,
as it usually appears in chalk to the eye, in the form of veins running
thro' its substance, must be considered only as an accidental or foreign
admixture; and, with respect to the minute portion of alkaline earth
which composed the remainder of the sediment, it cannot be supposed to
have been originally different from the rest, and incapable, from its
nature, of being converted into quick-lime, or of being dissolved in
water; it seems rather to have consisted of a small part of the chalk in
its mild state, or saturated with air, which had either remained, for
want of a sufficient fire to drive it out entirely, or had been
furnished by the distilled water.

I indeed expected to see a much larger quantity of sediment produced
from the lime, on account of the air which water constantly contains,
and with a view to know whether water retains its air when fully
saturated with lime, a lime-water was made as strong as possible; four
ounces of which were placed under the receiver of an air-pump, together
with four ounces of common water in a vial of the same size; and, upon
exhausting the receiver, without heating the vials, the air arose from
each in nearly the same quantity: from whence it is evident, that the
air, which quick-lime attracts, is of a different kind from that which
is mixed with water. And that it is also different from common elastic
air, is sufficiently proved by daily experience; for lime-water, which
soon attracts air, and forms a crust when exposed in open and shallow
vessels, may be preserved, for any time, in bottles which are but
slightly corked, or closed in such a manner as would allow free access
to elastic air, were a vacuum formed in the bottle. Quick-lime therefore
does not attract air when in its most ordinary form, but is capable of
being joined to one particular species only, which is dispersed thro'
the atmosphere, either in the shape of an exceedingly subtile powder, or
more probably in that of an elastic fluid. To this I have given the name
of fixed air, and perhaps very improperly; but I thought it better to
use a word already familiar in philosophy, than to invent a new name,
before we be more fully acquainted with the nature and properties of
this substance, which will probably be the subject of my further
inquiry.

It is, perhaps, needless to mention here, that the calcarious substances
used in making the above experiments should be of the purest kind, and
burnt with the utmost violence of heat, if we would be sure of
converting them into perfect quick-lime. I therefore made use of chalk
burnt in a small covered crucible with the fiercest fire of a
Black-smith's forge, for half an hour, and found it necessary to employ,
for this purpose, a crucible of the _Austrian_ kind, which resemble
black lead; for if any calcarious substance be heated to such a degree
in an ordinary or _Hessian_ crucible, the whole of it is melted down,
together with part of the vessel, into glass.

I now prepared to inquire into the properties of the caustic alkali; in
order to which, I made a caustic or soap ley in the following manner.

Twenty six ounces of very strong quick-lime made of chalk, were slaked
or reduced to a sort of fluid paste, with eleven pounds of boiling
water, and then mixed in a glass vessel with eighteen ounces of a pure
fixed alkaline salt, which had been first dissolved in two pounds and a
half of water. This mixture was shaken frequently for two hours, when
the action of the lime upon the alkali was supposed to be over, and
nothing remained but to separate them again from one another. I
therefore added 12 pounds of water, stirred up the lime, and, after
allowing it to settle again, poured off as much of the clear ley as
possible.

The lime and alkali were mixed together under the form of a very thick
milky liquor or fluid paste; because they are thus kept in perpetual
contact and equal mixture until they have acted sufficiently upon one
another: whereas in the common way of using a larger quantity of water,
the lime lies for the most part at bottom, and, tho' stirred up ever so
often, cannot exert its influence so fully upon the alkali, which is
uniformly diffused thro' every part of the liquor.

The above ley was found upon trial to be saturated by acids without the
least effervescence or diminution of weight.

It was now proper to examine whether the alkali suffered any loss in
becoming caustic, which I proposed to attempt by ascertaining the
strength of the ley, or the quantity of salt which a given portion of it
contained; from which by computation some imperfect knowledge might be
obtained of the quantity of caustic produced from the eighteen ounces of
mild salt.

I therefore evaporated some of my ley, but soon perceived that no
certain judgment could be formed of its strength in this way, because it
always absorbed a considerable quantity of air during the evaporation,
and the dried salt made a pretty brisk effervescence with acids, so that
the ley appeared stronger than it really was; and yet, upon proceeding
in the estimate from this rude and unfair trial, it appeared that the
salt had lost above a sixth in becoming caustic, and the quantity of
acid saturated by two drams of it was to the quantity of acid saturated
by two drams of salt of tartar, nearly as six to five.

These experiments are therefore agreeable to that part of the second
proposition which relates to the caustic alkali.

Upon farther examining what changes the alkali had undergone, I found
that the ley gave only an exceeding faint milky hue to lime-water;
because the caustic alkali wants that air by which salt of tartar
precipitates the lime. When a few ounces of it were exposed in an open
shallow vessel for four and twenty hours, it imbibed a small quantity of
air, and made a slight effervescence with acids. After a fortnight's
exposure in the same manner, it became entirely mild, effervesced as
violently with acids, and had the same effect upon lime-water as a
solution of an ordinary alkali. It likeways agrees with lime-water in
this respect, that it may be kept in close vessels, or even in bottles
which are but slightly covered, for a considerable time, without
absorbing a sensible quantity of air.

In order to know how much lime it contained, I evaporated ten ounces in
a small silver dish over a lamp, and melted the salt, after having
dissipated the water.[8]

The caustic thus produced was dissolved again in a small quantity of
water, and deposited a trifling portion of sediment, which I imagined at
first to be lime; but finding that it could easily be dissolved in a
little more water, concluded it to be a vitriolated tartar, which always
accompanies the fixed alkali of vegetables.

I then saturated the solution of the caustic salt with spirit of
vitriol, expecting thus to detect the lime; because that acid
precipitates a calcarious earth from its ordinary solutions. During the
saturation, a large quantity of white powder was formed; but this
likeways turned out to be a vitriolated tartar, which had appeared in
the form of a powder, because there was not enough of water in the
mixture to dissolve it.

Lastly, I exposed a few ounces of the ley in an open shallow vessel so
long, that the alkali lost the whole of its causticity, and seemed
entirely restored to the state of an ordinary fixed alkali; but it did
not however deposite a single atom of lime. And to assure myself that my
caustic ley was not of a singular kind, I repeated the same experiments
with an ordinary soap-ley, and with one made by mixing one part of a
pure fixed alkaline salt with three parts of common stone lime fresh
slaked and sifted; nor could I discover any lime in either. The first of
these contained a small quantity of brimstone, and was far from being
perfectly caustic, for it made a pretty brisk effervescence with acids;
but the last was so entirely deprived of its air, that it did not
diminish in the least the transparency of lime-water.

These experiments seem therefore to support the fourth proposition, and
to shew that the caustic alkali does not contain any lime.

As it seems probable, from the quickness and ease wherewith the alkali
was rendered caustic, that more lime had been employed than what was
just sufficient to extract the whole of its air, we are surprised to
find that little or none of the superfluous quick-lime was dissolved by
the water. But this _phaenomenon_ will become less surprizing, by
comparing it with some similar instances in chemistry. Water may be made
to deposite a salt, by the admixture of a substance which it attracts
more strongly than it does that salt; such as spirit of wine; and
quick-lime itself may be separated from water upon the same principle;
for if that spirit is added to an equal quantity of lime-water, the
mixture becomes turbid and deposites a sediment, which, when separated
and dissolved again in distilled water, composes lime-water. We may
therefore refer the above _phaenomenon_, with respect to the ley, to the
same cause with these, and say, that the water did not dissolve the
lime, because it already contained a caustic alkali, for which it has a
superior attraction.

I also rendered the volatile alkali caustic, in order to examine what
change it suffered in the operation, and obtained an exceedingly
volatile and acrid spirit, which neither effervesced with acids, nor
altered in the least the transparency of lime-water; and, altho' very
strong, was lighter than water, and floated upon it like spirit of wine.

I next inquired into the truth of the fifth proposition, in the
following manner.

Two drams of epsom-salt were dissolved in a small quantity of water, and
thrown into two ounces of the caustic-ley; the mixture instantly became
thick, like a decoction of starch or barley, by the magnesia, which was
precipitated. I then added spirit of vitriol by degrees, until the
mixture became perfectly clear, or the whole of the magnesia was again
dissolved; which happened without any effervescence or emission of air.

Half an ounce of chalk was dissolved in spirit of salt, the quantity of
which was so adjusted, that the mixture was not acid in the least
degree; and the solution was thrown into twelve ounces of the caustic
ley; which quantity I found, by experiment, to be sufficient for
precipitating almost the whole of the chalk. I now filtrated this turbid
liquor, and laid the powder remaining in the paper upon a chalk-stone,
in order to draw as much of the water from it as possible, and thereby
reduce it to the form of a more dense and heavy powder, that it might
subside the more perfectly in the following part of the experiment. I
then mixed it with about twenty ounces of pure water in a flask, and,
after allowing the powder to subside, poured off the water, which had
all the qualities of lime-water. And I successively converted eight
waters more into lime-water, seven of these in the same quantity, and
with the same management, as the first. The eighth was likeways in the
same quantity; but I allowed it to remain with the chalk, and shook it
frequently, for two days. This, after being filtrated, formed a cream or
crust upon its surface when exposed to the air; changed the colour of
the juice of violets into green; separated an orange-coloured powder
from a solution of corrosive sublimate; became turbid upon the addition
of an alkali; was entirely sweetened by magnesia; and appeared so
strong to the taste, that I could not have distinguished it from
ordinary lime-water. And when I threw some salt ammoniac into the lime
which remained, the vapour of the volatile alkali immediately arose from
the mixture.

In this experiment therefore the air is first driven out of the chalk by
an acid, and then, in order to separate this acid from it, we add an
alkali which has been previously deprived of its air; by which means,
the chalk itself is also obtained free of air, and in an acrid form, or
in the form of slaked lime.

We have also several processes for obtaining the volatile alkali in a
caustic form, which seem to be only so many methods of obtaining it in
its pure state, and free of fixed air. The first of these is the
separation of the alkali from an acid, merely by heat; an instance of
which we have from Mr. _Margraaf_.[9] He prepared from urine an
ammoniacal salt, the acid of which is the basis of the phosphorus, and
is of such a peculiar nature, that it endures a red heat without being
dissipated. Sixteen ounces of the neutral salt were subjected by him to
distillation. The acid remained in the retort, and he found in the
receiver eight ounces of an alkaline spirit, which, he tells us, was
extremely volatile, very much resembling the spirit of salt ammoniac
distilled with quick-lime; and no crystals were formed in it, when
exposed to the cold air.

A caustic volatile alkali may also be obtained, by mixing salt ammoniac
with half its weight of a caustic fixed alkali, or of magnesia which has
been previously deprived of its air by fire; and then submitting these
mixtures to distillation: Or merely by adding any ordinary volatile
alkali to a proper quantity of a caustic ley; for in this case the air
passes from the volatile to the fixed alkali, by a superior attraction
for the last, and, by a gentle heat, the compound yields a spirit
similar to that prepared from salt ammoniac and quick-lime.

It is therefore probable, that, had we also a method of separating the
fixed alkali from an acid, without, at the same time, saturating it with
air, we should then obtain it in a caustic form; but I am not acquainted
with an instance of this separation in chemistry. There are two indeed
which, at first sight, appear to be of this kind; these are the
separation of the fixed alkali from the nitrous acid by means of
inflamed charcoal, in the process for making _nitrum fixatum_, and of
the same alkali, from vegetable acids merely by heat; but, upon
examining the product of each process, we find the alkali either fully
or nearly saturated with air. In the first, either the charcoal or the
acid, or both together, are almost wholly converted into air; a part of
which is probably joined to the alkali. In the second, the acid is not
properly separated, but rather destroyed by the fire: a considerable
portion of it is converted into an inflammable substance and we learn
from Dr. _Hales_, that the bodies of this class contain a large quantity
of fixed air.

When we consider that the attraction of alkalis for fixed air is weaker
than that of the calcarious earths, and reflect upon the effects of heat
in chemistry, we are led to imagine, that alkalis might be entirely
deprived of their air, or rendered perfectly caustic, by a fire somewhat
weaker than that which is sufficient to produce the same change upon
lime; but this opinion does not seem agreeable to experience.

The alkalis do, however, acquire some degree of causticity in a strong
fire, as appears from their being more easily united with spirit of wine
after having been kept in fusion for some time. For that fluid, which
cannot be tinctured by a mild salt of tartar, will soon take a very deep
colour from a few drops of a strong caustic ley. The circumstances which
hinder us from rendering these salts perfectly caustic by heat, are
their propensity to dissipation in the utmost violence of the fire,
their extreme acrimony, and the imperfection of our common vessels. For
before the heat becomes very intense, the alkalis either evaporate, or
dissolve a part of the crucibles in which they are contained, and often
escape thro' their pores; which happens, especially as soon as they have
already acquired some degree of additional acrimony, by the loss of part
of their air.

The fusion also, which they so readily undergo, is well known by
Chemists, as a strong obstacle to the separation of the volatile from
the fixed parts of a compound by fire; accordingly, in several
processes, we are directed to add to the fusible compound some porous
substance which is incapable of fusion, and will retain the whole in a
spongy form, thereby to facilitate the dissipation of the volatile
parts.

In order to know whether an alkali would lose a part of its air, and
acquire a degree of causticity, when exposed, with this precaution, to
the action of a strong fire, I mixed an ounce and a half of salt of
tartar with three ounces of black-lead, a substance of any the most
unchangeable by chemical operations. This mixture I exposed, for several
hours, in a covered crucible, to a fire somewhat stronger than what is
necessary to keep salt of tartar in fusion. When allowed to cool, I
found it still in the form of a loose powder; and taking out one half, I
diluted it with water, and by filtration obtained a ley, which, when
poured into a solution of white marble in _aqua fortis_, precipitated
the marble under the form of a weak quick-lime: for the turbid mixture
gave a green colour to the juice of violets, and threw up a crust like
that of lime-water; and the precipitated powder collected and mixed with
salt ammoniac immediately yielded the scent of the volatile alkali.

Lest it should here be suspected, that the alkaline qualities of this
mixture, and of the precipitated marble, were not owing to a lime into
which the marble was converted, but to the alkali itself which was
added, it is proper to observe, that I mixed so small a proportion of
the ley with the solution of marble as made me sure, from certain
experiments, that the whole of the alkali was spent in performing the
precipitation, and was consequently converted into a neutral salt by
attracting the acid. The properties therefore of the mixture can only be
referred to a lime, as is indeed sufficiently evident from the crust
which is peculiar to lime-water.

I was therefore assured by this experiment, that an alkali does really
lose a part of its air, and acquire a degree of causticity, by the
proper application of heat; but finding by several trials, that the
degree of causticity which it had thus acquired was but weak, and that
the quick-lime produced in this experiment was exhausted and rendered
mild by a small quantity of water, I exposed the crucible together with
that half of the alkali which remained in it to a stronger fire, in
order to expel a larger quantity of air, and render it more remarkably
caustic; but the whole of it was dissipated by the force of the heat,
and the black lead, which still retained the form of a loose and subtile
powder, yielded little or nothing to water.

We learn then from the above experiment the reason why the alkali newly
obtained from the ashes of vegetables is generally of the more acrid
kinds of that salt. It never appears until the subject be converted into
ashes, and is supposed to be formed by the fire, and to be the result of
a particular combination of some of the principles of the vegetable; one
of which principles is air, which is contained in large quantity in all
vegetable matters whatever. But as soon as the smallest part of a
vegetable is converted into ashes, and an alkali is thus formed, this
salt necessarily suffers a calcination, during which it is kept in a
spongy form by the ashes, and shews a very considerable degree of
acrimony if immediately applied to the body of an animal but if the
ashes are for any time exposed to the air, or if we separate the alkali
from them by the addition of a large quantity of water and subsequent
evaporation, the salt imbibes fixed air from the atmosphere, and becomes
nearly saturated with it: tho' even in this condition it is generally
more acrid than salt of tartar, when this is prepared with a gentle
heat.

Borax has sometimes been referred to the class of alkalis, on account of
some resemblance it bears to those salts: but it has been demonstrated
by accurate experiments, that we should rather consider it as a neutral
salt; that it is composed of an alkali and of a particular saline
substance called the sedative salt, which adheres to the alkali in the
same manner as an acid, but can be separated by the addition of any acid
whatever, the added acid joining itself to the alkali in the place of
the sedative salt. As this conjunction of an acid with the alkali of
borax happens without the least effervescence, our principles lay us
under a necessity of allowing that alkali to be perfectly free of air,
which must proceed from its being incapable of union with fixed air and
with the sedative salt at the same time: whence it follows, that, were
we to mix the sedative salt with an alkali saturated with air, the air
would immediately be expelled, or the two salts in joining would produce
an effervescence. This I found to be really the case upon making the
trial, by mixing a small quantity of the sedative salt with an equal
quantity of each of the three alkalis, rubbing the mixtures well in a
mortar, and adding a little water. It is however proper in this place
to observe, that, if the experiments be made in a different manner, they
are attended with a singular circumstance. If a small quantity of the
sedative salt be thrown into a large proportion of a dissolved fixed
alkali, the sedative salt gradually disappears, and is united to the
alkali without any effervescence; but if the addition be repeated
several times, it will at last be accompanied with a brisk
effervescence, which will become more and more remarkable, until the
alkali be entirely saturated with the sedative salt.

This _phaenomenon_ may be explained by considering the fixed alkalis as
not perfectly saturated with air: and the supposition will appear very
reasonable, when we recollect, that those salts are never produced
without a considerable degree of heat, which may easily be imagined to
dissipate a small portion of so volatile a body as air. Now, if a small
quantity of the sedative salt be thrown into an alkaline liquor, as it
is very slowly dissolved by water, its particles are very gradually
mixed with the atoms of the alkali. They are most strongly attracted by
such of these atoms as are destitute of air, and therefore join with
them without producing an effervescence; or, if they expel a small
quantity of air from some of the salt, this air is at the same time
absorbed by such of the contiguous particles as are destitute of it, and
no effervescence appears until that part of the alkali, which was in a
caustic form or destitute of air, be nearly saturated with the sedative
salt. But if, on the other hand, a large proportion of the sedative salt
be perfectly and suddenly mixed with the alkali, the whole, or a large
part, of the air is as suddenly expelled.

In the same manner may we also explain a similar _phaenomenon_, which
often presents itself in saturating an alkali with the different acids:
the effervescence is less considerable in the first additions of acid,
and becomes more violent as the mixture approaches the point of
saturation. This appears most evidently in making the _sal diureticus_
or regenerated tartar: The particles of the vegetable acid here employed
being always diffused thro' a large quantity of water, are more
gradually applied to those of the alkali, and during the first additions
are chiefly united to those that are freest of air.[10]

That the fixed alkali, in its ordinary state, is seldom entirely
saturated with air, seems to be confirmed by the following experiment.

I exposed a small quantity of a pure vegetable fixed alkali to the air,
in a broad and shallow vessel, for the space of two months; after which
I found a number of solid crystals, which resembled a neutral salt so
much as to retain their form pretty well in the air, and to produce a
considerable degree of cold when dissolved in water. Their taste was
much milder than that of ordinary salt of tartar; and yet they seemed to
be composed only of the alkali, and of a larger quantity of air than is
usually contained in that salt, and which had been attracted from the
atmosphere: for they still joined very readily with any acid, but with a
more violent effervescence than ordinary; and they could not be mixed
with the smallest portion of vinegar, or of the sedative salt, without
emitting a sensible quantity of air.

As it now appeared that several alkaline substances have an attraction
for fixed air, I tried a few experiments to learn the relative strength
of their several attractions.

Twenty four grains of magnesia in fine powder were mixed with five
ounces of the caustic ley in a small vial, which was immediately corked
and shaken frequently for four hours. The ley was then poured off, and
the magnesia washed with repeated affusions of water, and dried. It had
lost about the half of its weight, and when reduced to a fine powder was
readily dissolved by acids with an effervescence which was hardly
perceivable: the alkali had therefore extracted its air. I also threw
some fresh magnesia into the ley which had been poured off, and thereby
rendered it perfectly mild and similar to a solution of salt of tartar;
so that it effervesced briskly with acids.

With an ounce of the mild spirit of salt ammoniac, I mixed a dram of
magnesia in very fine powder which had been previously deprived of its
air by fire; and observing that the magnesia had a tendency to concrete
into a solid mass, I shook the vial very frequently. After some days the
powder was increased to more than double its former bulk; and when the
vial was opened, the alkaline spirit emitted a most intolerably pungent
smell. It likewise floated upon water, but was not perfectly caustic;
for it still yielded some air when mixed with acids, and also rendered
lime-water turbid: neither of which would probably have happened if I
had used a greater quantity of magnesia, or had allowed the mixture to
remain a longer time in the vial. I now washed out the whole of the
mixture into a bowl, and dryed the magnesia until it lost all smell of
the alkali. It weighed a dram and fifty eight grains, effervesced
violently with acids, and therefore contained a large quantity of air,
which had been drawn from the alkali by a stronger attraction.

Having formerly shewn, that magnesia saturated with air separates an
acid from a calcarious earth, which it is not able to do after being
deprived of its air by fire; I now suspected that the air was the cause
of this separation, because I found that it was joined to the calcarious
earth at the same time that the acid was joined to the earth of
magnesia; and imagined that a pure calcarious earth might possibly have
a stronger attraction for acids than a earth of magnesia.

I therefore dissolved two drams of magnesia in the marine acid, and
thus obtained a compound of an acid and of the pure earth of this
substance; for the air which was at first attached to it, was expelled
during the dissolution. I then added thirty grains of strong quick-lime
in exceeding fine powder, shook the mixture well, and filtrated it. The
powder remaining in the paper, after being well washed, was found to be
a magnesia, which, as I expected, was destitute of air; for it was
dissolved by the vitriolic acid without effervescence. And the filtrated
liquor contained the lime united to the acid; for upon dropping spirit
of vitriol into it, a white powder was immediately formed.

We must therefore acknowledge a stronger attraction between the
calcarious earths and acids than between these and magnesia: but how
does it then happen, that, if magnesia saturated with air be mixed with
a compound of acid and calcarious earth, these two last, which attract
one another the most strongly, do not remain united; but the acid is
joined to the magnesia, and the calcarious earth to the air which it
attracts much more weakly than it does the acid? Is it because the sum
of the forces which tend to join the magnesia to the acid and the
calcarious earth to the air, is greater than the sum of the forces which
tend to join the calcarious earth to the acid, and the magnesia to the
air: and because there is a repulsion between the acid and air, and
between the two earths; or they are somehow kept asunder in such a
manner as hinders any three of them from being united together?

The first part of this supposition is favoured by our experiments, which
seem to shew a greater difference between the forces wherewith the
calcarious earth and magnesia attract fixed air, than between those
which dispose them to unite with the acid. The repulsions however hinted
in the second are perhaps more doubtful, tho' they are suggested in many
other instances of decomposition; but the bounds of my present purpose
will not allow me to enter upon this subject, which is one of the most
extensive in chemistry.

We meet also with a difficulty with respect to the volatile alkali
similar to the above. Thus a calcarious earth that is pure or free of
air has a much stronger attraction for acids than a pure volatile
alkali, as is evident when we mix quick-lime with salt ammoniac; for the
alkali is then immediately detached from the acid: and agreeably to this
I found, upon trial, that a pure or caustic volatile alkali does not
separate a calcarious earth from an acid. Yet, if we mix a mild volatile
alkali, which is a compound of alkali and air, with a compound of acid
and calcarious earth, these two last, which attract one another most
strongly, do not remain united; but the acid is joined to the alkali and
the earth to the air, as happens in the precipitation of a calcarious
earth from an acid, by means of the common or mild volatile alkali.

I remember likewise a parallel instance with regard to quick-silver.
This metal has an attraction for the vitriolic acid, and when joined to
it appears under the form of turbith mineral: but this attraction is
weaker than that of the fixed alkali for the same acid; for if we mix a
dissolved salt of tartar with turbith mineral, the turbith is converted
into a brown powder, and the alkali into vitriolated tartar; which
change happens the sooner, if the pure or caustic alkali is used. Yet,
if to a compound of quick-silver and the nitrous acid, we add a compound
of the fixed alkali and the vitriolic acid, or a vitriolated tartar, and
digest the mixture with a strong heat, the vitriolic acid does not
remain with the alkali, but is joined to the quick-silver which it
attracts more weakly, composing with it a turbith mineral; while the
alkali is joined to the nitrous acid which it likeways attracts more
weakly than it does the vitriolic, and is converted into salt-petre.

From some of the above experiments, it appears, that a few alterations
may be made in the column of acids in Mr. _Geoffroy's_ table of elective
attractions, and that a new column may be added to that table, according
to the following scheme, where the alkaline substances are all
considered as in their pure state and free of fixed air.


                Acids.                 Fixed air.
  ------------------------------    -----------------
           Fixed alkali,            Calcarious earth.
         Calcarious earth,           Fixed alkali.
   Volatile alkali and magnesia.       Magnesia.
                                     Volatile alkali.
       ----------------------        ---------------

At the foot of the first column several of the metals might follow, and
after these the earth of alum; but as I don't know what number of the
metals should precede that earth, I have left it to be determined by
further experience.

The volatile alkali and magnesia are placed in the same line of this
column; because their force of attraction seems pretty equal. When we
commit a mixture of magnesia and salt ammoniac to distillation, the
alkali arises and leaves the acid with the magnesia; because this earth,
by attracting the acid, represses its volatility, and it seems also to
diminish the cohesion of the acid and alkali, and to render them
separable by a gentle heat. If the magnesia be saturated with air, this
likewise, on account of its volatile nature and attraction for the
alkali, is driven up along with it, and makes it appear under a mild
form, and in the same manner do the alkali and air arise from a mixture
of salt ammoniac and of a crude calcarious earth.


Footnotes:

[1] June 5. 1755.

[2] Hoff. Op. T. iv. p. 479.

[3] Hoff. Op. T. iv. p. 500.

[4] Mr. _Margraaf_ has lately demonstrated, by a set of curious and
accurate experiments, that this powder is of the nature, and possesses
the properties, of the gypseous or selenitic substances. That such
substances can be resolved into vitriolic acid and calcarious earth, and
can be again composed by joining these two ingredients together. Mem. de
l'Acad. de Berlin. an. 1750, p. 144.

[5] Hoff. Op. T. iv. p. 480 & 500.

[6] Mem. de l'Acad. de Berlin. an. 1748, p. 57.

[7] Hoff. Op. T. iv. p. 480.

[8] This evaporation was performed in a silver dish, on account of the
acrimony of the salt; which is so very great, that, having once
evaporated a part of the same ley in a bowl of English earthen or stone
ware, and melted the caustic with a gentle heat, it corroded and
dissolved a part of the bowl, and left the inside of it pitted with
small holes.

[9] Mem. de l'Acad. de Berlin. an. 1746, p. 87.

[10] Boerh. Operat. Chem. process. LXXVI.


Transcribers Notes:

1. Author's spelling has been retained.

2. Minor punctuation issues have been corrected without note.





End of the Project Gutenberg EBook of Experiments upon magnesia alba,
Quicklime, and some other Alcaline Substances, by Joseph Black

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