THE SPACE RELATIONS OF ATOMS. 499 



But Buchner has prepared the dicarboxyl derivatives 

 according to all the known methods, and has found it 

 impossible to detect more than two isomers. This is the 

 number arrived at by arranging the H and R' atoms in two 

 planes one on each side of the ring and parallel with it ; 

 we may then have either one R' on one side of the ring and 

 one on the other, or both on the same side, (cis-trans iso- 

 merism) ; in no other way can the number of theoretical iso- 

 mers be limited to two. We must therefore accept a three- 

 dimensional arrangement for the trimethylene derivatives, 

 especially as the properties of Buchner's derivatives are such 

 as the " cis-trans " formulae would lead us to expect. And 

 from the great analogy between the trimethylene and the 

 hexamethylene dicarboxylic acids, which also exhibit the 

 maleicfumaric or cis-trans isomerism, we may extend this 

 arrangement to hexamethylene derivatives. Now these con- 

 clusions derive no support whatever from the optical evidence. 

 It must be remembered, however, that this evidence is purely 

 negative, and that, with the exception of benzene-deriva- 

 tives, ring-compounds have not been thoroughly investi- 

 gated as to their optical activity. In these circumstances 

 it is specially interesting to consider the case of limonene. 



This substance has a mixed methene-methine ring ; it 

 is undoubtedly active, and it has been supposed to contain 

 an asymmetric carbon atom. But, according to the recent 

 investigation of its constitution by Baeyer (Ber. xxvii., 439, 

 450), it contains no such atom. So that if Baeyer's limo- 

 nene formula hold its own .against its various rivals, we 

 have here the first exception to the empirical law that an 

 active molecule must contain an asymmetric atom. It is 

 certainly to be hoped that further investigation will either 

 supply the theoretical basis of this law, or bring the optical 

 into accord with the chemical evidence, and with the theory 

 of Sarrau. 



It is possible, however, that the inactivity of asymmetric 

 ring-compounds is only apparent. For even compounds 

 containing an asymmetric atom are often apparently inactive, 

 because like racemic acid they are composed of right- and 

 left-handed molecules in equal numbers. Such inactive 



35 



