LIVING SUBSTANCE 11 



teristics. Among these are the invariable presence- 

 of nitrogen along with the carbon, hydrogen, and 

 oxygen, and a very large and complex molecule, 

 which is always Isevo-rotatory, i.e. turns the rays of 

 polarized light to the left, ajid contains sometimes 

 thousands of atoms. 1 About half the weight is car- 

 bon and 15 per cent to 18 per cent nitrogen. 



A giant protein molecule is not exactly a unit in 

 itself, but is usually an aggregation of smaller atomic 

 groups or other protein molecules weakly held to- 

 gether by the bond of " chemical affinity," just as 

 a village of five hundred inhabitants may be thought 

 of as made up not exactly of that many individuals, 

 but of one hundred families composed in turn of 

 an average of five persons each. The atomic 

 groups may freely break away from the protein 

 molecule or be added to it, and in this instability lies 

 the great significance of the presence of the proteins 

 in all living matter. (See chapter on Metabolism.) 

 As examples of nearly pure protein may be mentioned 

 lean meat fiber and white of egg (albumen). 



J. Loeb and Paul! have called attention to the 

 strong probability that the proteins in animal proto- 

 plasm are united with the ionized 2 inorganic salts 

 to form "ion-protein compounds." This hypothesis 

 accounts for many otherwise inexplicable phenom- 

 ena, and explains the importance of even extremely 

 minute quantities of certain salts in life processes. 



1 An analysis of the blood pigment of the horse has yielded the fol- 

 lowing formula : C7i2Hn3o 



2 See page 10. 



