SECRETION OF URINE. 535 



happen, if there be a predominance of other acids in the urine, which will 

 seize upon its base, as soon as its own affinity for it is diminished by the 

 lowering of its temperature. By Dr. Prout it is believed that Lactic acid, 

 existing in the Blood or in the Urine in excess, is an ordinary source of this 

 deposit ; but the presence of this acid is altogether denied by Liebig ( 670). 

 The composition of Lithic Acid is as follows : 10 Carbon, 4 Hydrogen, 4 

 Nitrogen, 6 Oxygen. The amount of it usually excreted in the Urine of Man 

 is but very small ; it is occasionally, however, considerably increased ; but the 

 circumstances under which this increase takes place have not yet been exactly 

 determined. 



674. Uric acid is replaced in the Herbivorous animals by the Hippuric ; 

 the composition and properties of which are very different from those presented 

 by that substance. When pure, it forms long transparent four-sided prisms ; 

 it is soluble in 400 parts of cold water, and dissolves readily at a boiling heat ; 

 and it has a strong acid reaction, and bitterish taste. Its formula is 18 Carbon, 

 8 Hydrogen, 1 Nitrogen, and 5 Oxygen, with 1 equiv. of Water. It has very 

 curious relations with Benzoic acid ; which it yields, together with Benzoate 

 of Ammonia, when acted upon by a high temperature, or during the putrefac- 

 tion of the urine of which it forms a part. According to Liebig, the Hippuric 

 acid in the urine of the Horse and Ox is replaced by Benzoic acid, when the 

 animal is subjected to hard labour. It appears, from his recent experiments,* 

 that we are to regard Hippuric acid as a normal element of Human urine ; for 

 he has detected Benzoic acid among the products of its putrefaction ; and as 

 we know that the latter does not exist in the Urine of Man, and as there is no 

 other substance at the expense of which it can be formed during the putre- 

 factive process, we can scarcely hesitate to admit that such must be the case. 

 It is a very curious fact, that the introduction of Benzoic acid into the system 

 causes a large increase in the amount of Hippuric acid in the Urine ; and if this 

 be formed at the expense of the elements, which would otherwise have pro- 

 duced Uric acid, an easy method is pointed out for the elimination of the latter 

 substance from the blood, when it has accumulated there, the salts of Hippuric 

 acid being so much more soluble than those of the Uric. According to Keller,! 

 whose experiments were made upon himself, both Urea and Uric acid existed 

 in normal quantity in his urine ; whilst a large quantity of Hippuric acid was 

 being excreted ; whilst Mr. Alexander Ure states,J that he has succeeded, by 

 the administration of Benzoic acid, in preventing the deposition of Gouty con- 

 cretions, and even in removing them when they had been formed. 



675. Many remarkable changes are effected in Lithic acid, by the operation 

 of other chemical agents ; and these changes are very important, in their bear- 

 ing on Pathological conditions of the Urine. When Uric acid is subjected to 

 the action of Oxygen, it is first resolved into Urea and a compound termed 

 Jilloxan. Now this Alloxan, when acted on by a new supply of Oxygen, is 

 resolved into Urea and Oxalic acid ; or, with a still further amount of Oxygen, 

 into Urea and Carbonic Acid : a fact, which has a very important bearing 

 on the production of Calculi composed of Uric and Oxalic acids, and which 

 explains the remarkable alterations which are often observed in the layers of 

 these concretions. It is affirmed by Liebig, that the calculi which are com- 

 posed of Urate of Ammonia, or of Oxalate of Lime, occur in persons, in whom, 

 from want of exercise, or from other causes, the quantity of Oxygen intro- 

 duced into the system is beneath what it ought to be. When patients suffer- 

 ing under Uric acid Calculi take more exercise, the Urates are replaced by 

 Oxalates, in consequence of the larger amount of Oxygen introduced into the 



* Loc. cit. t Liebig's Animal Chemistry, [Am. ed. p. 325.] 



* Medico-Chirurgical Transactions, vol. xxiv. 



