INTRODUCTION. . 15 



be tetravalent, for in the first case four of the six affini- 

 ties of the two carbon atoms are saturated by hydrogen, 

 in the latter only two. 



Compounds, in which all the affinities of the carbon 

 atoms thus united are saturated by other elements, are 

 called saturated ; compounds which possess free affini- 

 ties, non-saturated. 



Experience has shown that only such non-saturated 

 carbon compounds can exist in an isolated condition, 

 in which two, four, or in general terms an even num- 

 ber of affinities, are unsaturated. Atomic groups, in 

 which an odd number of affinities are unsaturated, 

 cannot be isolated. 



It is, however, questionable whether, with the excep- 

 tion of carbonic oxide CO, non-saturated compounds 

 of carbon . are really capable of existence. In the so- 

 called non-saturated compounds the carbon atoms are 

 probably united with each other with more than one 

 affinity. Ethylene C 2 H 4 may be regarded as the satu- 

 rated compound of a group of two carbon atoms, which 

 are united with each other by means of two affinities, 



( CH 2 

 as \ || It is not positive proof of the contrary 



( CH 2 . 



that this body conducts itself in most reactions as a 

 non-saturated compound, and, for instance, unites with 

 the greatest ease with two atoms of a monovalent 

 element or monovalent group, as in this reaction the 

 double union of the carbon atoms can be broken up 

 and the simple union, quite sufficing for the suste- 

 nance of the atoms in their position, re-established. 

 According to this view the carbon atoms in acetylene 

 C 2 H 2 must be united with each other by means of three 

 of their free affinities each. 



In a large number of compounds, especially the so- 

 called aromatic bodies, we are compelled to admit that 

 the union of the carbon atoms takes place in a manner 

 different from that mentioned above; that, for the 

 purpose of holding together the single atoms of carbon, 

 more than one of the affinities of each of the .carbon 

 atoms is employed. Benzol C 6 H 6 , for instance, accord- 



