8o THE CHEMICAL CONSTITUTION OF THE PROTEINS 



13. Corrections for the Solubilities of the Phosphotungstates. 

 Although the conditions of precipitation are not always the same, the 

 variation is not sufficient to cause significant change in the solubilities 

 of the phosphotungstates. When precipitated from a volume of 

 200 c.c. the following corrections should be added, and the sum of the 

 figures for amino nitrogen and non-amino nitrogen subtracted from 

 the figures for the monamino acids : 



This long method has been rigidly tested by Van Slyke upon pure 

 amino acids singly, and when mixed together, and upon typical proteins 

 for which he gives the following values expressed in percentages of the 

 total nitrogen : 



The accuracy of the method is borne out by the figures under the 

 column of total nitrogen ; except in one case, the nitrogen recovered 

 is within I per cent. In the duplicate analyses of these proteins the 

 maximum differences were 2-14 per cent, for the histidine (edestin) and 

 I -60 per cent for the monoamino nitrogen (hair). These differences 

 were more than twice any other deviations from the figures in the 

 series. The highest difference was 079 per cent, for histidine and the 

 lowest 0*05 for cystine. 



