i28 POPULAR SCIENCE MONTHLY 



attached an atom of hydrogen, but as carbon is usually considered 

 quadrivalent, a fourth seemingly unused bond of affinity remains free 

 to each of these carbon atoms. This affinity may be considered as the 

 residual affinity. To this substance Kekule has assigned the structure 

 illustrated by the graphical formula : 



H 



The introduction of three double linkings between the alternate 

 pairs of carbon atoms satisfies the demands for quadrivalence in these 

 atoms. But Kekule clearly called attention to the fact that a sort of 

 equilibrium existed between all the carbon atoms, such that the pres- 

 ence of one of the three double linkings between any two adjacent car- 

 bon atoms, when both were involved in the formation of a derivative, 

 would not necessarily change the properties of the derivative from that 

 one formed when two adjacent carbon atoms were united by only a 

 single bond. Xow the Kekule formula, and in fact all the older 

 formula? assigned to this compound, represent only particular phases 

 in the motions of the molecule. The space-formula proposed by 

 Collie, 2 in which the atoms are represented as in a state of continual 

 vibration, serves well for the basis of our modern conception. Upon 

 examination of the absorption spectrum of benzol we note the presence 

 of seven distinct bands all quite similar and closely situated with 

 reference to each other, appearing between the oscillation frequencies 

 3,725 and 4,200. These bands are in that part of the ultra-violet spec- 

 trum where the absorption bands due to keto-enol tautomerism displayed 

 themselves. At once the idea of a similar make-and-break of linkings 

 between the carbon atoms suggested itself, and, in exact accordance 

 with this hypothesis, the seven distinct bands may find here their cause 

 of formation. 



In keto-enol (aliphatic) tautomerism an even number of carbon 

 atoms is always involved in the make-and-break of linkings. Accord- 

 ingly with the benzol molecule we may assume that two, four or six 

 carbon atoms may enter into this phase at one time. If the carbon 

 atoms are numbered consecutively from 1 to 6, we should have in 

 order the following conditions which represent the change of linkings 

 between certain numbered carbon atoms: (1 and 2), (1 and 3), (1 

 and 4), (1 and 2, with 3 and 4), (1 and 2, with 3 and 5), (1 and 2, 

 with 4 and 6), (1 and 2, with 3 and 4, with 5 and 6). At the outset 

 we shall suppose the benzol ring to be elastic and capable of under- 



2 Chem. Soc. Trans., 71, 1013, 1897. 



