72 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Except lysine, the natural amino acids all react quantitatively in 

 five minutes : half an hour is required for this substance. Ammonia 

 and methylamine require one and a half to two hours, purines and 

 pyrimidines two to five hours, and urea eight hours. 



The completeness of the reaction may be tested by repeating the 

 process and ascertaining if more gas is evolved. 



Glycine and cystine evolve a larger volume of gas than the theoret- 

 ical ; the factor -926 may be used in estimating cystine ; 3 per cent 

 of the total volume of gas must be deducted for glycine. 



Klein has since constructed a simpler and more permanent apparatus 

 for this estimation ; it is reproduced in fig. 3. 



FIG. 3. 



Reproduced by kind permission from the Journal of Biological Chemistry. 



