674 Mr. N. P. McCleland on the 



unless the group is asymmetrical with respect to the plane, 

 and is sufficiently unsaturated to make for stability. 



8. A few remarks of an entirely speculative nature as to 

 the structure of benzene may be added. 



Many structural formulae have been proposed for the 

 benzene ring, a critical summary of which has been given 

 by Dr. Stewart in his book on Stereochemistry. 



The chief conditions to be fulfilled may be summarized as- 

 follows ; — 



1. The 6 carbon atoms are equivalent. 



2. The 6 hydrogen atoms likewise. 



3. The laws of position isomerism must be complied with. 



4. The existence of derived substances such as naphthalene 



must be allowed for. 



5. A reason must exist for the law of substitution, which 



appears to demand that the carbon atoms exist in two 

 sets of three, those in the same group being in the 

 meta position to each other. 



6. The hydrogens attached to ortho carbons are nearer to 



one another than those attached to carbons in meta 

 or para positions. 



7. The reduction of benzene to cyclohexane must bo 



possible in a simple manner. The carbon atoms in 

 the latter substance must be in the same cyclic order 

 as they were in benzene. 



Further, cyclohexane is very similar to other naphthenes,. 

 and it would appear that the carbon atoms in this substance 

 are in a plane. 



A structural formula which appears to comply with all 

 necessary conditions is the following, based on the " Kekule 

 ring ,J and the octahedron formula, suggested first, I believe, 

 by Professor Collie. The carbon atoms are arranged at 

 the corners of an octahedron, formed by alternately raising 

 and depressing the 6 carbons of the Kekule ring from 

 the plane of the ring. The electrons forming the bonds 

 between carbons are supposed to revolve about centres 

 in the mean plane; though it is not possible to state that the 

 orbits are entirely in this plane, it appears that they are very 

 nearly so. The electrons forming the free valency of each 

 carbon revolve in planes (almost) parallel to the main plane,, 

 and the hydrogen atoms are disposed outside the carbons. 

 This model closely resembles that suggested by Barlow and 

 Pope on crystallographic grounds. 



