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XL VII. Van't HofFs Hypothesis and the Constitution of 

 Benzene. By J. E. Maesh, B.A* 



THE hypothesis of Van't Hoff is based on the fundamental 

 principle that the four valencies of the carbon atom are 

 directed from the centre of a regular tetrahedron towards the 

 angles. Without entering into any detailed explanation, I 

 may be allowed briefly to notice that from this fundamental 

 principle follows the isomerism of bodies containing a carbon 

 atom combined with four different groups (called an asym- 

 metric carbon atom), and this isomerism is found actually to 

 exist, and is characterized by possession by the two isomeric 

 compounds of enantiomorphous crystalline form and opposite 

 rotatory power. The two compounds are in fact represented 

 by formulae in space related to one another as the object to 

 its image, the object being different from and not superposable 

 on its image, though the dimensions of both are the same. 



Further, there follows from the fundamental principle that 

 there is an isomerism of a different nature from the preced- 

 ing among bodies which contain more than one asymmetric 

 carbon atom, an isomerism not characterized by rotatory 

 power, an example of which is found in the case of racemic 

 and inactive mesotartaric acids. 



Again, there follows an isomerism in the case of two 

 doubly linked carbon atoms, each combined with two different 

 groups, an isomerism called " geometrical isomerism," and 

 " alloisomerism," of which a familiar example is to be found 

 in the case of maleic and fumaric acids ; and all the above 

 classes of isomerism, well established by numerous examples, 

 are inexplicable by the old structural formulae as expressed 

 on a plane. 



CH 2 



R 2 <f X CH 2 



H 2 C CHCsH 7 



X NH 



Fig. 1 (Conine). 



There is further another class of isomerisms, not of par- 

 ticularly recent observation, but one which I wish to deal 

 with in more detail, as it appears to entail a slight modifica- 

 tion or rather extension of the original conception of the 

 theory. 



* Communicated by the Author, having been read before the British 

 Association Meeting at Bath, 1888. 



