Lecture Xv.

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.