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 _phænomenon_ 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 _phænomenon_, 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 _phænomenon_ 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 _phænomenon_, 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.