432 Mr. J. E. Marsh on 



isomers in general would be identical. Such isomers in the 

 case of benzene derivatives we are not acquainted with ; we 

 might have expected, for instance, that salicylic aldehyd, 

 which contains two different substituting groups in the ortho 

 position, and which is also a body occurring in nature, would 

 have possessed rotatory power, and would have had an isomer 

 of equal but opposite rotatory power. 



But it may be that the isomerism in question, not being due 

 to an asymmetric carbon atom, is one which is not charac- 

 terized by the same properties as those whose isomerism is 

 due to such an atom, and that the two isomers have always 

 been obtained as a mixture in equivalent proportions that we 

 know no means of separating them, that if separated they 

 possess identical properties in most respects, and that we 

 know of no special property by which they may be dis- 

 tinguished. 



In conclusion, it remains to be noticed that the constitution 

 of benzene, as established by von Baeyer, or as I have en- 

 deavoured to modify it, is one which is peculiar to benzene 

 and its substitution derivatives ; it disappears in the case of 

 additive derivatives when the ordinary single and double 

 linkings take its place ; a saturation of any two carbon atoms, 

 as in dihydroterephthalic acid, or as in quinone, at once 

 annihilates the peculiar character of the benzene ring. 



Hence it will be obvious that naphthalene and phenanth- 

 rene must be excluded from the category of benzene deriva- 

 tives. The constitution of naphthalene, as composed of two 

 benzene nuclei having two carbon atoms in common, breaks 

 down because it is impossible for two benzene nuclei to have 

 any of their carbon atoms in common. The free affinities 

 cannot in fact be directed at the same time towards the 

 centres of two hexagons. 



CH OH 



/ \/\ 

 CH C CH 



I II I 

 CH C CH 



CH CH 



Fig. 10. 



Thus, without altering the original formula of naphthalene, 

 we must alter our conception of the body and regard it not 

 as a benzene derivative but as analogous to the unsaturated 

 fatty compounds with six single and five double linkings 

 (figs. 10 and 11). 



