340 INFECTION AND IMMUNITY. 



tion more tangible through an analogy which was 

 drawn with the so-called ring or nucleus of benzol 

 and its side-chains. The molecule of benzol, C 6 H 6J 

 has a definite formation in which each carbon 

 atom is linked to two others in such a manner as 

 to form a ring; three valences of each carbon 

 atom are satisfied in this way, and the fourth is 

 satisfied by atoms of hydrogen, one of which is at- 

 tached to each carbon atom, thus : 



H 



I 

 c 



H-C ^ C H 



II I 

 H C C H 



V 



This ring is analogous to the "Leistungskern" of 

 the cell. A great variety of chemical compounds 

 exists and very many may be produced syntheti- 

 cally by substituting for one or more atoms of 

 hydrogen, one or more other groups of atoms which 

 may be very simple or very complex. The groups 

 which have been substituted are called side-chains. 

 Thus benzoic acid is formed from benzol by sub- 

 stituting the acid radical CO OH for a particular 

 H, and the COOH in this instance is a side-chain 

 of the ring : 



o 



c 



A 



H C ^CH. 



II I 



H-C C-H 



x // 



C 



H 



