THE ELIMINATION OF METABOLIC PRODUCTS. 593 



dissolves completely on heating, and appears again on cooling. On stand- 

 ing for some time, crystals often appear in the sediment, which will not 

 dissolve on heating the urine. These crystals are free uric acid, while the 

 sediment was monosodium urate. The precipitation of the latter is, 

 partly at least, due to the cooling of the urine, for it is much more soluble 

 in hot water than in cold. As the crystals form, the acidity of the urine 

 decreases. The question arises whether the change in the reaction of the 

 urine has any connection with the deposition of the urate precipitate. We 

 have at a previous place mentioned the insolubility of uric acid; 1 it requires 

 at 18 C., 39,000 parts of water to dissolve one part of the acid. The 

 values given in the literature are often widely different from the above 

 value which is taken from the work of His. This is partly explained by 

 the fact that it is usually disregarded that glass, especially common glass, 

 contains alkali, which it gives up to water which is in contact with it, and 

 this affects the analysis. His, again, has called attention to the great 

 tendency of uric acid to form supersaturated solutions. The acid urate 

 of sodium, also called the mono-urate, is far more soluble in water than 

 uric acid itself. Now this urate is deposited frequently in urine, and of 

 other cases free uric acid is found in the sediment, and, in fact, so much in 

 it that it is hard for us to believe that it was present as such in the urine. 

 Camerer 2 compared the solution of uric acid in the urine with the following 

 experiments. He mixed a saturated solution of acid urate of sodium, 

 which reacted alkaline toward litmus, with a solution of acid phosphate 

 of sodium. The mixed solution showed an acid reaction and was perfectly 

 clear at 37 C. On cooling the mixture, the reaction toward litmus 

 changed. The solution became alkaline and uric acid was precipitated. 

 A chemical decomposition had taken place. From the acid phosphate of 

 sodium (NaH 2 PO 4 ), and at the cost of the sodium in the monosodium 

 urate, disodium phosphate (Na 2 HPO 4 ) had been formed, and uric acid 

 had been set free; which, on account of its insolubility, was precipitated. 

 On heating the solution, the reverse process took place, and the reaction 

 of the urine became acid. In the urine there is always more or less alkali 

 phosphate present, which may have the same effect as in the above test- 

 tube experiment. Naturally, according to this explanation, it must be 

 assumed that the uric acid is originally present as the monosodium salt. 

 There is no question that part of the uric acid is actually present in this 

 form; but, on the other hand, it is certain that a part of the uric acid 

 is present, combined in some other way. This is evident from the fact 

 that from a simple solution of alkali urate, the whole of the uric acid may 

 be precipitated by the addition of acid, while this is not the case with 

 urine. A part of the uric acid remains in solution after the urine has 



1 Cf. T. Paul: Pharm. Ztg. 1900, also Lecture XIII, p. 298. 



2 Deut. med. Wochschr. 17, No. 10, p. 356 (1896). 



