This salt, when properly prepared, is a _bi-carbonate_, but so
delicately are the affinities of its constituent parts balanced, that
the application of a very moderate temperature is sufficient to subvert
them, and to occasion partial decomposition. Mr. Phillips states that
although he has seen what he believes to be real bi-carbonate in the
state of the moist crystals, yet he has never met with any that was dry
which had not lost one-fourth of its carbonic acid by exposure to heat;
it is then a white gritty powder, less soluble in water than the
sub-carbonate, like which it possesses an alkaline taste, and turns
vegetable yellows brown, but both in a less degree. This salt, which is
generally sold, as the carbonate of the pharmacopœia, and the
bi-carbonate of chemists, Mr. Phillips considers as a compound of an
atom of carbonate, (_sub-carbonate_) and an atom of bi-carbonate,
combined with four atoms of water. It is therefore, according to the
phraseology of some chemists, a _Sesqui-carbonate_,[637] as being equal
to an atom and a half of acid and one atom of base. The chemical
habitudes of this salt, as connected with its medicinal applications,
are similar to those of the _carbonate of potass_, which see. MED. USES.
As it is less nauseous, so is it more eligible than the _sub_-carbonate
of the same alkali; in other respects its effects are the same; _vide
Sodæ Sub-carbonas_. DOSE, grs. x to ʒss.[638] ADULTERATIONS. If the
salt, after super-saturation with dilute nitric acid, give a precipitate
with nitrate of baryta, it contains some sulphuric salt; and if with
nitrate of silver, we may infer the presence of a muriate.