CHAP, xxxiv.] URIC ACID. 501 



known as lithate of ammonia, although Lehmann, Becquerel, and Heintz, all 

 agree that it is composed principally of lithate of soda. 



In the urine of the carnivora, uric acid is present in small quantity, but, as 

 a general rule, it is absent from the urine of the herbivora ; and, curiously 

 enough also, it cannot be detected in the urine of the omnivorous pig. The 

 excrement of birds, and that of serpents and other reptiles, and of many 

 insects, contains a large quantity of alkaline urates. Guano, as is well known, 

 is chiefly composed of lithate of ammonia. 



After profuse perspiration, the quantity of uric acid has been found to be 

 diminished in the urine ; but a purely animal diet exerts but little influence 

 upon the quantity of this substance excreted by the kidneys. It is much 

 increased, however, in all febrile conditions of the system, and after imperfect 

 digestion of food. In cases where the respiratory function is impaired, the 

 amount of uric acid has been found to be abnormally increased ; and insuf- 

 ficient exercise will produce a similar effect. 



Uric acid has been detected in the blood of healthy men by Garrod, and in 

 considerably increased proportion in the blood of gouty patients. It has 

 also been detected in the perspiration, and the deposits formed about the 

 joints of gouty persons are largely composed of it. 



According to Wohler and Frerichs, the introduction of lithic acid into the 

 blood is followed by an increased secretion of urea and oxalate of lime in the urine, 

 a point of considerable interest, when we know that, by the influence of peroxide 

 of lead, a similar decomposition of the lithic acid may be induced artificially. 



When all these circumstances are considered, more especially that, in cer- 

 tain instances in which the respiratory changes are not carried on with the 

 activity consistent with perfect health, a greatly increased quantity of lithic 

 acid is eliminated by the kidneys, there appears ample evidence to show, that 

 lithic acid is one of the purely excrementitious substances derived from the 

 disintegration of the tissues, and formed by the action of oxygen upon effete 

 material. By a process of further oxidation, the lithic acid itself becomes 

 converted into urea as we just now mentioned. 



Hippuric Acid (C 18 H 8 N0 5 , HO), according to Liebig, exists in small quan- 

 tity in healthy human urine ; but it is obtained in considerable quantity from 

 the urine of horses, cows, and other herbivorous animals. It is quite in- 

 odorous, has a rather bitter taste, is slightly soluble in cold, but very soluble 

 in hot water and alcohol, characters in which it differs from uric acid. It is 

 easily prepared from the urine of cows by precipitation by hydrochloric acid, 

 and subsequent purification. It is, however, absolutely necessary that the 

 urine should be perfectly fresh, otherwise the hippuric acid will be found to 

 have been entirely converted into benzoic acid, a change which may also be 

 induced in the pure acid by the action of heat and mineral acids. It has been 

 stated by Mr. Ure, that if benzoic acid be taken, it is eliminated from the 

 system as hippuric acid. 



Hippuric acid has been found in the urine of many herbivorous animals, 

 and by Lehmann in that of the tortoise ( Testudo Grceca) and many herbivorous 

 insects. It is not present in the urine of the carnivora. In cases of diabetes, 

 it is stated by the same observer to be never absent from the urine ; and in 

 health may usually be detected if the diet be purely of a vegetable character. 

 This acid, like uric acid, must be looked upon as an excrementitious substance, 

 and plays no other part in the system. 



