1914] A. W. Thomas 221 



precipitants do not in most cases completely precipitate the Compounds 

 with which they combine, but have been chosen because they seemed to 

 be the most adequate for the purpose. The Compounds which they 

 precipitate are often precipitated by other metaUic salts used in the 

 scheme; for instance, cytosine, while isolated here by the use of am- 

 moniacal silver nitrate sol., is also precipitated to some extent by am- 

 moniacal lead acetate sol. Care must be taken not to confuse crystals 

 of calcium sulphate with crystals of the hydrochlorides or other salts of 

 the purine bases, for it has been our experience that calcium sulphate 

 often appears in the final Solution to be used for the Identification of 

 these Compounds. Since phosphotungstic acid is a general precipitant 

 for organic bases, some of which may also be precipitated by the other 

 metallic salts used in the Isolation of the hexone bases, care must be 

 exercised that salts of other organic bases are not confused with the 

 crystalline salts of arginine or histidine." 



The accompanying outline (pp. 218-9), from the Bulletin by 

 Schreiner and Shorey, in 191 1 (see foot-note 3 of this paper), will 

 be found very useful in working out the detailed scheme given 

 above. 



Laboratory of Biological Chemistry of Columbia University, 

 College of Physicians and Surgeons, New York 



