CONSTITUTION OF ORGANIC COMPOUNDS. 287 



In a similar manner, carbon atoms unite, forming chains, as, for 

 instance : 



II III I I I I 



_ C C , C C C , C C C C , etc. 



II III till 



The above carbon chains have 6, 8, and 10 free affinities, respect- 

 ively, which may be saturated by the greatest variety of atoms or 

 radicals. The chain combination of carbon, above indicated by the 

 first three members of a series, may, as far as is known, be continued 

 indefinitely. This fact, in connection with the possibility of saturat- 

 ing the free affinities with various atoms or radicals, indicates the 

 almost unlimited number of possible combinations to be formed in 

 this way. In fact, the existence of such an enormous number of 

 carbon compounds is greatly due to the property of carbon to form 

 these chains. 



It is not always the case that the atoms when forming a chain are 

 united by one affinity only, as above, but they may be united by two 

 or three affinities, as indicated by the compounds C 2 H 4 and C 2 H 2 , the 

 graphic formulas of which may be represented by 



H 



Finally, it is assumed that the carbon atoms are united partially 

 by double and partially by single union, as, for instance, in the so- 

 called closed chain of C 6 , capable of forming the saturated hydrocarbon 

 benzene, C 6 H 6 : 



H 





& 



\ H/ So/ \H 



1 A 



A chain has also been termed a skeleton, because it is that part of an organic 

 compound around which the other elements or radicals arrange themselves, 

 filling up, as it were, the unsaturated affinities. 



Homologous series. This term is applied to any series of organic 

 compounds the terms or members of which, preceding or following 

 each other, differ by CH 2 . Moreover, the general character, the con- 

 stitution, and the general properties of the members of an homologous 

 series are similar. 



The explanation regarding the formation of an homologous series 

 is to be found in the above-described property of carbon to form 



