_Of the Phosphoric Acid._
The most important acid of _animal_ origin, though it has lately been
found in some mineral substances, is the _phosphoric_.
Phosphorus itself is a remarkable substance, much resembling sulphur,
but much more inflammable. It has been procured chiefly, till of late,
from urine, but now more generally from _bones_, by means of the
vitriolic acid, which unites with the calcareous earth of which bones
consist, and sets at liberty the phosphoric acid, or the base of that
acid, with which it was naturally combined. The acid thus procured,
mixed with charcoal, and exposed to a strong heat, makes phosphorus.
This substance burns with a lambent flame in the common temperature of
our atmosphere, but with a strong and vivid flame if it be exposed to
the open air when moderately warm. In burning it unites with the
dephlogisticated air of the atmosphere, and in this manner the purest
phosphoric acid is produced.
This acid is also procured in great purity by means of the nitrous or
vitriolic acids, especially the former, which readily combines with the
phlogiston of the phosphorus, and thus leaves the acid pure. In this
process phlogisticated air is produced.
This acid is perfectly colourless, and when exposed to heat loses all
its water, and becomes a glassy substance, not liable to be dissipated
by fire, and readily uniting with earths.
United to the mineral alkali, it forms a neutral salt, lately introduced
into medicine. United to the mineral and vegetable alkalis naturally
contained in urine, it has obtained the name of _microcosmic salt_,
frequently used as a flux for mineral substances with a blow-pipe.
Besides the phosphoric, there are other acids of an animal origin; as
that of _milk_, that of _sugar of milk_, that of the _animal calculus_,
and that of _fat_.
The acid of milk is the sour whey contained in butter-milk, which, by a
tedious chemical process, may be obtained pure from any foreign
substance.
The sugar of milk is procured by evaporating the whey to dryness, then
dissolving it in water, clarifying it with whites of eggs, and
evaporating it to the consistence of honey. In this state white crystals
of the acid of sugar of milk will be obtained.
By distilling these crystals with nitrous acid, other crystals of the
proper _acid of sugar of milk_ will be obtained, similar to those of the
acid of sugar.
If the human calculus be distilled, it yields a volatile alkali, and
something sublimes from it which has a sourish taste, and therefore
called the _acid of the calculus_. It is probably some modification of
the phosphoric acid.
Animal fat yields an acid by distillation, or by first combining it with
quick-lime, and then separating it by the vitriolic acid. Siliceous
earth is corroded by this acid.