THE PROTEINS 3 



probability of proteins, as we differentiate them at 

 present, being chemical units, but since many of the 

 crystalline plant proteins show a constancy of proper- 

 ties and ultimate composition there is little reason 

 for the assumption that these at least are mixtures of 

 two or more individuals. 



Concerning the size of the protein molecule some 

 idea may be gained when it is recalled that the molec- 

 ular weight has been calculated to be approximately 

 15,000. 



The proteins possess the property of turning the 

 plane of polarized light to the left, the degree of rota- 

 tion for an individual protein varying with the solvent 

 employed. 



CLASSIFICATION OF PROTEINS 



At present proteins are classified according to their 

 physical properties, as, for example, their solubility 

 in pure water, weak salt solutions and dilute acids and 

 alkalies. It is well recognized that such a classifi- 

 cation is far from ideal, but it is the most satisfactory 

 plan that has been offered. When more complete 

 knowledge is gained concerning the chemical make-up 

 of the protein molecule a classification will undoubt- 

 edly be framed which will be based upon the presence 

 or proportion of certain chemical groups in the differ- 

 ent proteins. 



All albuminous substances may be divided into 



