_Sub-carbonate of Soda._
QUALITIES. _Form_, octohedrons, truncated at the summits of the
pyramids; it effloresces when exposed to the air, and at 150° Fah.
undergoes watery fusion, its crystals containing as much as seven
proportionals of water; _Taste_, mild, alkalescent. CHEMICAL
COMPOSITION. Soda 29·5—carbonic acid 20·7. SOLUBILITY. It is soluble in
two parts of water at 60°, and in considerably less than its weight of
boiling water; it is insoluble in alcohol. INCOMPATIBLE SUBSTANCES are
enumerated under _Potassæ Carbonas_. MED. USES, are similar to those of
the sub-carbonate of potass, but it is preferable to it for internal
use, as being more mild and less nauseous; and moreover Fourcroy states
it as his opinion that soda is more eligible for medicinal purposes than
potass, on account of its analogy with animal substances, which always
contain it, while on the contrary, no portion of potass is found in
them. Sir Gilbert Blane assumes an opposite opinion, and observes that,
as far as he can judge of the comparative powers of the two fixed
alkalies, he should greatly prefer Potass to Soda, as a remedy for
gravel, one reason of which he thinks may be found in the fact that the
Soda is an element of the animal fluids, since it enters largely into
the composition of bile, so that it is more likely to be arrested in the
course of the circulation and diverted from the urinary organs. A
gentleman, says Sir Gilbert, who was subject to frequent fits of gravel,
and in the habit of making experiments on the small concretions which he
passed, found that Soda dissolved them, but that Potass did not;
nevertheless he experienced sensible relief, and even temporary cure,
from the internal use of the latter alkali, but no benefit from the
former. Are then the absorbents more disposed to take up soda than
potass? The results of experience do not appear to sanction such a
conclusion. FORMS OF EXHIBITION. It may be administered in solution, in
an electuary, or in pills; when exhibited in the latter form, it must be
previously deprived of its water of crystallization, (_Sodæ Sub-carbonas
exsiccata_. L.) or the pills will fall into powder as they dry; unless
where the water of crystallization is essential to the formation of the
pill, as to that of _Pill: Ferri comp_. DOSE, gr. x to ʒj, twice or
thrice a day. See _Form. 28, 143, 144_.