vii KATABOLIC CONSTITUENTS OF URINE 387 



into a solution of sodium carbonate, acid sodium urate is obtained 

 (C 5 H 2 NaN 4 3 ). On dissolving it in caustic alkali, the second atom 

 of hydrogen is replaced, and the neutral urate (C 5 H 2 Na 2 N 4 3 ), 

 which is highly unstable, is formed. 



Both uric acid and the acid urates are soluble with difficulty 

 in water : 6-7 litres of water are required to dissolve 1 grin, uric 

 acid at body temperature (Bunge). This would lead us to suppose 

 that the urates are present in urine in the form of neutral salt. 

 But against this conjecture must be set the fact that the urine, 

 which is clear and acid when passed, forms on cooling to the 

 temperature of the air, especially in winter, the sedimentum 

 lacteritium, which contains large crystals of uric acid coloured dull 

 yellow by the red pigment of the urine. The crystalline sediment 

 dissolves in heating the urine to body temperature. C. Voit and 

 Hofmann demonstrated that this fact depends on the presence in 

 the urine of acid sodium phosphate, which when the urine cools 

 is converted into neutral phosphate,' thus removing the bases from 

 the urates, and causing the precipitation and crystallisation of 

 free uric acid. On the formation of the sediment the acidity of 

 the urine falls and rises again when the sediment re-dissolves, 

 the uric acid being once more converted into acid urate. 



As regards the origin of uric acid, it was formerly held that 

 it arose from incomplete oxidation of the cleavage products of the 

 proteins (Liebig). The following facts can be mustered in support 

 of this view : (a) by means of oxidising agents uric acid can 

 readily be converted into urea and carbonic acid outside the 

 body; (6) when introduced into the body of a dog it almost 

 entirely reappears in the urine in the form of urea (Zabelin, 

 Frerichs, Wohler) ; (c) when mixed with the blood and circulated 

 in the liver, it is converted into urea (Ascoli). 



These facts, however, do not prove that urea is normally 

 formed by direct oxidation of uric acid as shown in the last 

 paragraph. On artificially obstructing respiration in dogs, cats, 

 and rabbits (Senator), or on bleeding these animals (Naunyn, 

 Riess), no perceptible increase of uric acid is obtained. 



It is a striking fact that the ingestion of ammonium salts 

 increases the production of uric acid in birds (v. Schroder), and 

 that urea injected into the portal vein is partly converted into 

 uric acid (Meyer and Jaffe). After excising the liver in geese, 

 a diminution of uric acid and corresponding increase of ammonia 

 and lactic acid has been observed in the urine, which seems to 

 show that in birds the liver is able to form uric acid synthetically 

 from ammonia and lactic acid (Minkowski). On the other hand, 

 it has been shown that ammo-acids (leucine, glycocoll, aspartic 

 acid), which give rise to a formation of urea in mammals, also 

 give rise to uric acid in birds (Knieriem). 



As regards mammals and man, however, no data can be 



