226 Biological Chemistry. 



For the estimation of the diamino-acids, arginine, 

 histidine, and lysine, the protein is hydrolyzed with sul- 

 phuric acid, which is afterwards quantitatively precipitated 

 with barium hydroxide. The liquid is then acidified with 

 sulphuric acid, and phosphotungstic acid is added. This 

 latter substance possesses the property of precipitating 

 various basic substances, such as alkaloids, which are 

 carried down in the form of phosphotungstates. The 

 precipitate is then decomposed by shaking with barium 

 hydroxide in excess, with the formation of barium phos- 

 photungstate, which is insoluble, and can be filtered oif. 

 The excess of baryta in the solution is then removed 

 by carbon dioxide or sulphuric acid added in the exact 

 amount required, and the filtrate from the barium salt, 

 which contains the free diamino-acids, is then con- 

 centrated. These are then separated from one another 

 by taking advantage of the facts that arginine can be 

 precipitated as a silver salt in a solution made strongly 

 alkaline with barium hydroxide, histidine can be pre- 

 cipitated as a silver salt in a nearly neutral solution, 

 whereas lysine cannot be precipitated as a silver salt, but 

 is obtained from the mother liquors after the separation of 

 the two other substances, first in the form of its phospho- 

 tungstate, which latter is afterwards converted into the 

 picrate. The process, which has been devised by Kossel 

 and his pupils, is a somewhat elaborate one, and has only 

 been outlined above, and gives, with careful work, satis- 

 factory results. 



The substance tryptophan, which, from a biological 

 point of view, is very important, is destroyed by hydrolysis 

 with acids. In order to obtain it the protein must be 

 hydrolyzed by a ferment ; to estimate the amount set free 

 colorimetric and other methods have been suggested. 



In the following table the percentages of the various 



