THE PRINCIPLES OF ORGANIC CHEMISTRY 47 



also combine with itself. This fact lies at the foundation of 

 Kekule's law of the linking of atoms, which is one of the main 

 foundation stones of modern organic chemistry. The genesis 

 of this idea of Kekule's was singular. He tells us that it 

 came to him more or less as a dream. As he was sitting half 

 asleep by the fire, he seemed to see the atoms executing a 

 mazy dance, till suddenly some of them separated themselves 

 into chains, while others joined themselves in rings. He sat 

 up all night working out the consequences of this dream. 

 Very briefly it came to this, that if we consider a single carbon 

 atom with its tetrad valency, exercising a power of combination 

 with four atoms of hydrogen or their equivalent, it may be 



I 



symbolically written thus, C ; if another atom joins itself 



I 



to this, a compound will be formed with a skeleton structure 



of this kind, viz., and so on. Each of the vacant 



c 



I 



' bonds,' as they may be termed, can be combined with 

 hydrogen or other elements, and we can easily see that as we 

 go on adding carbon atoms, for each carbon atom two hydrogen 

 atoms or their equivalent can be also added. Thus we get 

 what is known as an homologous series. Supposing the bonds 

 in the above case to be combined with hydrogen, we obtain 

 the series, C n H 2n + 2 ; this is the series of paraffin hydro- 

 carbons, the initial member of which is : 



Methane, C H 4 , followed by 

 Ethane, C 2 H 6 , 

 Propane, C 3 H 8 , 

 Butane, C 4 H 10 , etc. 



If two adjacent carbon bonds in such a chain be left 



