HIPPU'RIC ACID (from Gk. trroc, hippos, horse + oi5'pov, ouron, urine), A com pound of great interest, loth to the chemist and to the physiologist. It derives its name from its having been first discovered in the urine of the horse, and that fluid, or the renal secretion of the cow, affords us the best and readiest means of obtaining it. The fresh urine is boiled with a slight excess of milk of lime and filtered; the filtrate is evaporated to a small volume, cooled, and acidified with hydrochloric acid, when hip puric acid separates out in the forum of fine needle-like crystals. When obtained by a proc ess of slow crystallization, the crystals of hippu ric acid are moderately large, at first colorless. but subsequently becoming milk-white, four sided prisms, which are devoid of odor, but have a faintly hitter taste. They dissolve readily in boiling water and in spirit. but are only sparing ly soluble in cold water and in ether. Its chemi cal name is benzovl-amido-acetic acid. CACO. and it may be prepared artificial ly from benzoic acid, CA.Ctill, and glyein, NILCH,C0011. When boiled with strong min eral acids, hippuric acid takes up water and splits up again into its chemical components, ben zoic acid and glycin. It is a normal constitu ent of the urine of the horse, cow, sheep. goat, hare, elephant, etc., and most probably is to be found in the urine of all vegetable feeders. In normal human urine, if the food is an ordinary mixed diet it occurs in very small quantity, but is increased by an exclusively vegetable diet, and in the well-known disease diabetes: Although hippuric acid usually occurs in mere traces in human urine, we can artificially produce it at will in the body, and cause it to be eliminated in comparatively large quantity. If we swallow
benzoic acid, it seems to take up the elements of glycin in its passage through the system. and thus to form hippuric acid, which appears abun dantly in the urine. Hippuric acid is formed in the animal body not only from benzoic acid itself. but from any substance (e.g. kinic acid) that may be readily transformed into benzoic acid. Some such substances are contained in grass, hay, and in many berries, and are also found among the products of the putrefaction of pro teids, especially those of vegetable origin. In carnivora the formation of hippuric acid has been shown to take place chiefly in the kidneys; in the herbivore, however. benzoic acid is largely transformed into hippuric acid even if the kid neys have been removed. In. birds the ingestion of benzoic acid causes the formation not of hip puric, but of cenanthylic acid. The hippuric acid which occurs in the animal organism exists in combination with bases, chiefly as hippurate of soda and hippurate of lime. The last-named salt can be obtained by the mere evaporation of the urine of the horse.