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SCIENCE PROGRESS 



pairing with their neighbours. It will be seen that this formula 

 is not greatly different from that originally suggested by J. J. 

 Thomson, it is symmetrical and contains no ethylenic linkages. 

 The author elaborates a theory to explain the phenomena of 

 substitution in the benzene ring based on the influence of the 

 electronic environment of the first substituting group which 

 causes the free electrons on each side of it to pair up. The 

 movement of any two electrons from their mean position in 

 order to pair up will disturb the electrical equihbrium of the 

 system, and the others will move in such a manner as to restore 

 it. Thus in [xvii] and [xviii] the influence of a method and 

 nitro-group is indicated. The argument that certain groups 



H H 



[xvii] 



[xviii] 



such as methyl should attract the electrons and others such as 

 the nitro should repel them is ingenious, but too long to be 

 given here. The consequence of this arrangement, according 

 to the author, is that the positions occupied by the electrons 

 are to a great extent closed to substitution ; hence in [xvii] 

 o-/)-substitution will occur, while in [xviii] m-substitution takes 

 place. An attempt is made to explain the fact that o- and 

 />-substitution in many cases do not take place to an equal 

 degree, but here perhaps the author is less happy. The theory 

 is also applied to other ring structures such as pyridine, thio- 

 phene, furane, and pyrrol with considerable success. 



The important work of Henri on the absorption spectrum 



