642 OF SECRETION. 



acids, which are thereby rendered soluble. It is in this manner that he partly 

 explains the usually acid reaction of healthy urine ; the other causes of which 

 will be presently noticed. If there be an undue proportion of Lithic acid in 

 the urine, it will be precipitated on cooling ; because it is less soluble in a 

 cold than in a warm solution of phosphate of soda ; and the same result will 

 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, ex- 

 isting 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 ( 846). 

 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. 



a. 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 dis- 

 so.lves 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 putrefaction 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 sub- 

 stance at the expense of which it can be formed during the putrefactive process, we can 

 scarcely hesitate to admit that such must be the case. It is a very curious fact, that the intro- 

 duction 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 other- 

 wise have produced 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,f whose experiments 

 were made upon himself, both Urea and Uric acid existed in normal quantity in his urine, 

 though 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 de- 

 position of Gouty concretions, and even in removing them when they had been formed. 



b. Many remarkable changes are effected in Lithic acid, by the operation of other chemi- 

 cal agents; and these changes are very important, in their bearing 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 Jllloxan. Now thisAlloxan, 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 i'act, which has a very important bearing on the production 

 of Calculi composed of Uric and Oxalic acids, and which explains the remarkable alterna- 

 tions which are often observed in the layers of these concretions. It is affirmed by Liebig, 

 that the calculi which are composed 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 suffering under Uric 

 sirid Calculi take more exercise, the Urates are replaced by Oxalates, in consequence of the 

 larger amount of Oxygen introduced into the system; and if the oxygenation could be carried 

 still further, the latter would cease to be deposited, their elements passing off in the form of 

 Urea and Carbonic acid. These views are borne out by the results of Lehmann's experi- 

 ments upon himself; for he found that the violent exercise, which raised the proportion of 

 Urea in the urine by. more than one-third ( 844) brought down the amount of Uric acid^ 

 from 1 - 18 to -042, or nearly one-half. 



c. Another change is that which gives rise to the peculiar compound termed JlUantoin; 

 which naturally exists in the fluid of the Allantois of the fotal calf. This may be formed 

 artificially by boiling Uric acid with peroxide of lead ; from which process there result an 



* Loc. cit. f Liebig's Animal Chemistry, p. 327. 



J Medico-Chirurgical Transactions, vol. xxiv. 



