x PREFACE TO SECOND EDITION 



Our methods of analysis of the proteins have been extended 

 and improved by Van Slyke, who, in conjunction with Levene, 

 has also introduced a method for the investigation of what is 

 termed the "leucine" fraction. 



There were only a few gaps in the section devoted to the 

 synthesis of the amino acids ; one has been completely filled by 

 Pyman, who has synthesised histidine. 



In this new edition a more detailed account of the methods 

 of analysis of the proteins has been given. The omissions 

 of the previous edition which have been kindly pointed out to 

 me have been inserted, and it is hoped that this edition will 

 have a larger usefulness than the first. 



My thanks are due to Mr. H. J. Page for his help in read- 

 ing the proofs. 



R. H. A. P. 



January, 1912. 



