The free spirit of inquiry, and taste for experimental researches
which have of late so happily prevailed, have given rise to several
very important discoveries in the course of the last half century;
among which, those of the properties of the electrical fluid, and
of fixed air, hold a principal rank. That excellent philosopher and
experimentalist, the late Doctor Hales, first proved that most
bodies contain, as one of their component principles, a quantity
of air, differing from that of the common atmosphere in several of
its properties. The proportion of this element varies in different
substances, and in some constitutes nearly one half of their weight.
All calcareous earths, the testaceous powders, Magnesia and alkaline
salts contain it abundantly, and have the strongest affinity with it of
any bodies, except metals. From all these it is discharged by an acid,
and the stronger the acid is, the more sudden and plentiful is the
discharge of this vapour, which is of the same nature as that emitted
by fermenting liquors. Though it may be inspired in small quantities
with impunity, and, as appears from some late observations, even with
advantage in some cases,[o] yet it is a known fact that animals expire
sooner in a receiver filled with fixed air, than in vacuo. Lastly, it
is said to have the remarkable property of rendering putrid substances
sweet.
[o] The reader is referred for further information on this subject, to
an excellent Treatise on the Management of Pregnant and Lying-in Women,
lately published by my worthy and ingenious friend Mr. White; to a
volume of Experiments and Observations, by Dr. Percival, which is now
in the press; and to some very curious papers on factitious air, which
have lately been communicated to the Royal Society, by Dr. Priestley.
In the precipitation of Magnesia, the acid of the Epsom salt uniting
with the alkali which is added to the solution, expels the fixed air
which the latter contains; but the vapour instead of being dissipated
as in the common effervescing mixtures of acids and alkalis, is quietly
and immediately absorbed by the Magnesia earth, to which it has a
strong affinity; and thus a double elective attraction takes place.
The very large proportion of this element contained in Magnesia Alba
has given Dr. Black an opportunity of throwing much light on this
hitherto obscure subject; and that excellent chemist has drawn such
inferences from the result of his experiments on Magnesia, Quick-lime,
and other alkaline substances, as are of the utmost importance to
chemistry. As this treatise may fall into the hands of several who may
not have an opportunity of perusing the valuable Essays in which these
experiments are contained, I shall take the liberty to make a short
extract from them, in order to elucidate the subject of the subsequent
pages.
The first object of Dr. Black's enquiry was, whether Magnesia could
be reduced to Quick-lime. To this purpose, he calcined an ounce of
Magnesia in a strong fire. When taken out of the crucible and weighed,
it had lost seven twelfths of its weight. Combined with different
acids, it formed salts of a similar nature to those constituted by the
same acids with the uncalcined powder; but dissolved in them without
the least degree of effervescence.
It slowly precipitated the corrosive sublimate of mercury, in the form
of a black powder; whereas before calcination a dark red precipitate
was formed from the same substance. Mixed with a warm solution of salt
ammoniac, it separated the volatile alkali from the acid; but it made
no separation of an acid from a calcareous earth, nor did it induce
any change upon lime-water; whereas in an uncalcined state, it rendered
quick-lime mild. Lastly, being digested for some hours with water, it
produced not the least alteration in the water.[p]
[p] In making this experiment some time since, I imagined that Doctor
Black had been mistaken in this point, and that some impurity in
the water had prevented the success of his process, for I found the
water impregnated, as I supposed, with the Magnesia. Flushed with my
supposed success, I proceeded to make experiments on the lithontriptic
powers of this water, which I found to be very considerable, acting
more efficaciously on the human calculus, than either oyster-shell
lime water, or a dilute solution of soap ley. I communicated this
interesting intelligence to some of my medical friends; but on
repeating my experiment several times with different parcels of
Magnesia, that the truth of the fact might be absolutely ascertained,
I at last was convinced, to my no small mortification, that what I had
too sanguinely flattered myself to be a discovery likely to be highly
serviceable to mankind, was founded on error; and that the properties
communicated to the water proceeded from some calcareous matter which
the Magnesia had received by being washed with impure water. I mention
this as a caution to every young experimentalist, to be extremely
careful in drawing conclusions. However, as something may be learned,
even from an unsuccessful experiment, it proves that a very small
quantity of lime is sufficient to impregnate a large quantity of water;
for I used the calcined Magnesia, in the same proportion as lime is
directed for making lime-water, so that very little of it could be
quick-lime. And as oyster-shell lime water is a superiour solvent of
the calculus to the water prepared with stone lime, is there not some
reason to think that the calcareous earth, which has been dissolved in
hard water, may, when calcined, be a more powerful lithontriptic, than
either of the others? If any inference can be drawn from it, which may
in the least promote the interests of mankind, I shall be sufficiently
recompensed for the humiliating circumstance of recounting an erroneous
experiment.
In pursuing his inquiries, he found that a very small portion of what
had been lost in the calcination was water, and that the other part
was fixed air, by the loss of which the Magnesia was deprived of its
power of effervescing with acids. And from hence the Doctor concluded,
that the change made in calcareous substances and in Magnesia by
calcination, was chiefly produced by depriving them of this air; and
that this volatile Proteus may be conveyed from one body containing it
to another body with which it has a greater affinity.
Thus lime being deprived of its air by calcination, and having a
stronger affinity with it than alkaline salts have, being mixed with a
lixivium of these salts, absorbs all the air from them, deprives them
of their property of effervescing with acids, and renders them more
acrid, at the same time that the lime becomes mild, and incapable of
impregnating water, but recovers its power of fermenting when mixed
with an acid.