This acid, the name of which (horse) is derived from its abundance in the urine of the horse, is closely connected with the aromatic benzoic acid. It can, in fact, be made to appear in the urine of man by the administration of benzoic acid with the food. Its chemical formula is C9H9NO3, and it contains 7'8 per cent of nitrogen. When separated from the urine it appears in the forms presented above (fig. 136), which represent large, white, four-sided prisms or elongated needles easily soluble in hot water and in alcohol.
The copious excretion of hippuric acid observed in animals fed on meadow hay is due to the herbs mixed with the grass, but the precise plants from which it is derived do not appear to have been ascertained. It appears in maximum quantities, 2 ozs. per diem or more, in animals fed on wheat and oat straw, but little is found in the urine of those fed on oats or corn, or on bean straw. It seems probable that it is formed in the liver and intestines by the union of glycocine with substances belonging to the benzoic acid series, or this may possibly occur in the kidney itself.
Hippuric acid, which sometimes constitutes as much as 2 per cent of the urine of the horse, is usually combined with calcium and sodium, forming hippurates of those metals.