THE PROTEINS 11 



substances. It is usually accepted at the present time 

 that the peptones are relatively simple polypeptides, 

 the line of demarcation between a simple peptone and 

 a complex peptide not being well defined. 



THE AMINO ACIDS 



For nearly a century chemists have been seeking 

 to establish the composition and structure of the pro- 

 tein molecule. Progress, which was slow and irregular 

 in the earlier decades of this period, has taken rapid 

 strides in the last twenty years, more intimate knowl- 

 edge of the problem being gained during this inter- 

 val than in all previous time. The investigation has 

 been pursued in three directions first the demolition 

 of the molecule and the subsequent identification of 

 the resulting fragments; second, the determination of 

 the quantitative relationships of these fragments ; and 

 finally, attempts to unite the disintegration products 

 in such a manner as to reproduce the original molecule. 



After a considerable period of investigation it was 

 established that, although the protein molecule may 

 yield different types of substances according to the 

 character of the means employed for disrupting it 

 thus indicating a variety of possible lines of cleavage, 

 hydrolysis furnishes the most promising types of 

 units. Latterly, this type of chemical reaction has 

 been employed exclusively and it has yielded the 

 important information now available concerning the 

 nature of the protein decomposition products. Each 



