THE COMPOSITION AND CHARACTERS OF THE URINE 1167 



(1) WEYL'S TEST. On treating a solution of creatinine with a small quantity of 

 very dilute sodium nitroprusside solution and then with weak caustic alkali, a rich 

 ruby red colour is produced which gradually changes to yellow. On now adding acetic 

 acid and warming, the solution becomes green and then blue, and finally a precipitate 

 of Prussian blue is formed. 



(2) JAFFE'S TEST. On treating a solution of creatinine with a few drops of a 

 watery solution of picric acid and dilute caustic potash, an intense red colour is produced. 

 This reaction is made use of for the quantitative estimation of creatinine in the urine. 



Like the other nitrogenous constituents, creatinine can be regarded 

 as partly exogenous, partly endogenous in origin. The exogenous part is 

 derived from the creatine contained in meat. When meat is excluded from 



FIG. 531. Creatmine. 



FIG. 532. Uric acid. (FUNKE.) 



the diet, the output of creatinine becomes remarkably constant and is little 

 affected by the total amount of proteins taken, the amount excreted during 

 starvation being practically identical with that excreted on a full protein 

 diet. It is said to be increased during febrile conditions and as a result of 

 violent muscular exercise. 



URIC ACID. Uric acid is 2-6-8-trioxypurine. 



HN CO N=C(OH) 



OC C NH, 



HN- 



or 



>CO 



(HO)C C NHs 



II II 

 N C N 





C(OH) 



Uric acid forms small rhombic crystals. The crystalline form varies 

 considerably in the presence of impurities. The different forms of uric 

 acid crystal which may occur in the urine are shown in the accompanying 

 figure (Fig. 532). It is extremely insoluble in pure water, one part of uric 

 acid requiring 39,000 parts of water at 18 C. for its solution. It is easily 

 soluble in concentrated sulphuric acid and alkalies. 



It may be prepared from human urine or from guano, which consists almost entirely 

 of urates. In order to prepare it from guano, this is dissolved with the aid of heat in 

 dilute sodium carbonate, filtered, and the filtrate treated with a few drops of concen- 

 trated hydrochloric acid and boiled. On allowing to cool, the uric acid crystallises out. 



