CONSTITUTION OF ORGANIC COMPOUNDS. 247 



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

 respectively, which may be saturated by the greatest variety of 

 atoms or residues. The chain combination of carbon, above 

 indicated by the first three members of a series, may, as far as 

 it is known, be continued indefinitely. This fact, in connection 

 with the possibility of saturating the free affinities with various 

 atoms or residues, 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 H 



\c=cc , H CEC H. 



^ \H 



It is finally assumed that the carbon atoms are united par- 

 tially by double and partially by single union, as, for instance, 

 in the so-called closed chain of C 6 , capable of forming the satu- 

 rated hydrocarbon benzine, C 6 H 6 : 



\ 



X ' 



H 



H 



( II I II 



W H/ \J>' >H 



. ' 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 un- 

 saturated 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 constitution, and the general properties of the 

 members of an homologous series are similar. 



