CHEMICAL BASIS OF THE ANIMAL BODY. 1249 



not a normal constituent of mammalian muscle but is found in 

 the muscles of some fishes, and has been obtained from sweat. 

 It crystallizes in colourless prisms or tables according to the 

 conditions under which the separation takes place and the mode 

 of preparation, and frequently, owing to imperfect development, 

 the crystals assume a very characteristic ' whetstone ' form. 



Fig. 205. Creatinine Crystals. (Krukenberg after Kiihne.) 



Creatinine is readily soluble in cold water (1 in 11*5) also 

 in alcohol, but is scarcely soluble in ether. The aqueous solu- 

 tions are usually alkaline, but some observers regard the alka- 

 linity as due to impurities. It acts as a powerful base, forming 



Fig. 206. Creatinine-Zinc-Chloride Crystals. (Krukenberg after Kiihne.) 



compounds with acids and salts which crystallize well. Of 

 these the most important is the salt with chloride of zinc 

 (C 4 H 7 N 3 0) 2 ZnCl 2 , both on account of its characteristic crystal- 

 line form and of its general insolubility in comparison with the 

 other compounds of this substance. Hence its formation is 

 employed not merely for the determination of creatinine but for 



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