[Note A: As these salts have only been understood of late, they have
not, properly speaking, any old names. Mr Morveau, in the First Volume
of the Encyclopedia, calls them _Mephites_; Mr Bergman gives them the
name of _aerated_; and Mr de Fourcroy, who calls the carbonic acid
_chalky acid_, gives them the name of _chalks_.--A]
SECT. XVII.--_Observations upon Carbonic Acid, and its Combinations._
Of all the known acids, the carbonic is the most abundant in nature; it
exists ready formed in chalk, marble, and all the calcareous stones, in
which it is neutralized by a particular earth called _lime_. To
disengage it from this combination, nothing more is requisite than to
add some sulphuric acid, or any other which has a stronger affinity for
lime; a brisk effervescence ensues, which is produced by the
disengagement of the carbonic acid which assumes the state of gas
immediately upon being set free. This gas, incapable of being condensed
into the solid or liquid form by any degree of cold or of pressure
hitherto known, unites to about its own bulk of water, and thereby forms
a very weak acid. It may likewise be obtained in great abundance from
saccharine matter in fermentation, but is then contaminated by a small
portion of alkohol which it holds in solution.
As charcoal is the radical of this acid, we may form it artificially, by
burning charcoal in oxygen gas, or by combining charcoal and metallic
oxyds in proper proportions; the oxygen of the oxyd combines with the
charcoal, forming carbonic acid gas, and the metal being left free,
recovers its metallic or reguline form.
We are indebted for our first knowledge of this acid to Dr Black, before
whose time its property of remaining always in the state of gas had made
it to elude the researches of chemistry.
It would be a most valuable discovery to society, if we could decompose
this gas by any cheap process, as by that means we might obtain, for
economical purposes, the immense store of charcoal contained in
calcareous earths, marbles, limestones, &c. This cannot be effected by
single affinity, because, to decompose the carbonic acid, it requires a
substance as combustible as charcoal itself, so that we should only make
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