118 PHYSIOLOGICAL CHEMISTRY 



the proportion between the two is very constant, their relative quan- 

 tity in the same individual being nearly always 



Uric acid 1 part. 



Urea 45 parts. 



Uric acid is a colorless, crystallizable substance, very slightly soluble 

 in cold or hot water, insoluble in alcohol and in ether. It is less easily 

 decomposed than urea, remaining for a long time unchanged under 

 ordinary conditions. If treated with concentrated sulphuric acid it is 

 decomposed, with the production of ammonia and carbonic acid. If 

 boiled with dilute nitric acid, it dissolves with a yellow color and 

 abundant liberation of gas-bubbles ; and, on evaporation, the solution 

 leaves a brilliant red stain, which is changed to purple by the addition 

 of ammonia water. This is known as the " murexide test" for uric 

 acid or the urates. 



Uric acid, like urea, is formed within the body by the metamorphosis 

 of nitrogenous organic substances. It is most abundant under the use 

 of animal food, is diminished by a vegetable diet, and is reduced to 

 a minimum, though it does not entirely disappear, during complete 

 abstinence. It is also increased by muscular exercise and diminished 

 by repose. It is this substance which indirectly causes the acid reaction 

 of the urine. It is nowhere present normally in a free form, being by 

 itself exceedingly insoluble ; but simultaneously with its production it 

 unites with part of the alkaline base of the phosphates, thus becoming 

 sodium urate, which is soluble and neutral in reaction, and giving 

 rise to sodium biphosphate, which communicates to the urine its acid 

 reaction. 



11. Sodium Hippurate, C 9 H 8 N0 3 Na. 



This is also a saline body, formed by the union of sodium with a 

 nitrogenous organic acid, namely, hippuric acid (Cs,H 9 N0 3 ), so called 

 because first discovered in the urine of the horse. It is comparatively 

 abundant in most herbivorous animals, especially the horse, ox, sheep, 

 goat, elephant, camel, and rabbit ; while it is absent, or nearly so, in the 

 carnivorous animals. In human urine, under an ordinary mixed diet, 

 it is constantly present, amounting to about 0.35 gramme per day, or 

 about one-half the quantity of uric acid. It increases perceptibly under 

 a vegetable diet, and diminishes or disappears under the exclusive use 

 of animal food. It thus alternates in quantity, under these circum- 

 stances, with uric acid. In the urine of the horse, which normally 

 contains hippuric acid, after continued abstinence from food, this sub- 

 stance ceases to appear and uric acid takes its place. Herbivorous 

 animals, when deprived of food, are placed in the condition of carnivora, 

 since the ingredients of the urine must then be derived from the meta- 

 morphosis of their own substance. In the calf, while living on the 

 milk of its dam, the urine contains uric acid ; after the animal is 

 weaned and begins to live on vegetable food, the uric acid disappears, 

 and the urine contains salts of hippuric acid. 



