CRYSTALLIZABLE NITROGENOUS MATTERS. Ill 



requires a corresponding supply of nitrogenous food ; and the final 

 result of the internal metamorphosis of such substances is mainly repre- 

 sented by the excretion of urea. 



Sodium Urate, C 5 H 3 N 4 3 Na. 



As its name indicates, this is a saline body, consisting of a nitro- 

 genous organic acid, namety, uric acid (C 5 H 4 N 4 3 ), in union with so- 

 dium. A portion of it is also in combination with potassium, but the 

 sodium salt is in much the greater quantity of the two. The urates 

 are found normally only in the urine, where they exist in the proportion 

 of about 1.45 parts per thousand. The entire quantity of uric acid 

 excreted by a healthy, full-grown man, is about 0.1 gramme per day. 

 It is, therefore, very much less abundant than urea; and, according 

 to the researches of J. Ranke, the proportion between them is very con- 

 stant, the relative daily quantity of the two substances in the same 

 individual being nearly always 



Uric acid . . . . . 1 part. 

 Urea 45 parts. 



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

 soluble in either cold or hot water, quite insoluble in alcohol and in 

 ether. It is much less easily decomposed than urea, remaining for a 

 long time unchanged under all 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 dis- 

 solves 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 a drop 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, and diminished by a vegetable diet, and is reduced to a 

 minimum, though it does not entirely disappear, during complete absti- 

 nence. It is this substance which indirectly, in great measure, 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 bases of the phosphates, 

 thus becoming mainly sodium urate, which is soluble and neutral in 

 reaction, and giving rise to sodium biphosphate, which communicates 

 to the urine its acid reaction. 



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, C 9 H 9 N0 3 , so called 

 because it was first discovered in the urine of the horse. It is com- 

 paratively abundant in the urine of most herbivorous animals, especially 

 the horse, ox, sheep, goat, elephant, camel, and rabbit; while it is 



