498 



SCIENCE PROGRESS. 



again have two inactive isomers. Representing the direction 

 of the rotation for each group by R for right-handed and 

 L for left-handed, these are 



I. 



Thus for C0 2 H(CHOH) 4 C0 2 H we have besides active 

 isomers the bodies mucic and allomucic acid (Fischer, Ber. 

 d. chem. Ges. xxv., 1247 and xxiv., 2136), both of which 

 cannot be rendered active. 



In short, wherever similar asymmetric groups are 

 opposed to one another we have at least one permanently 

 inactive substance. 



We have now exhausted the cases which may be sup- 

 posed to come within one or other of our three classes of 

 exceptions ; but there still remains a host of substances 

 unaccounted for — substances possessing the formula 

 AR'R"R'"and yet inactive. In the first place, among all 

 the great number of known ring-compounds, none except 

 such as possess an asymmetric carbon-atom is known to 

 possess optical activity. Yet the evidence goes to show 

 that, unlike benzene, many of these compounds have three- 

 dimensional molecules. This follows from Buchner's 

 observations on the derivatives of trimethylene. In this, 

 the simplest of the ring-compounds, the carbons of the ring 

 must lie in one plane, because there are only three of them. 

 Now Buchner {Ann. Chem. (Liebig), cclxxxiv., 197-226) 

 has shown that the radicals attached to the carbon-atoms 

 do not lie. in this plane, for otherwise there must be three 

 isomeric disubstituted tri-methylenes, CH 2 CHR' ' CHR' *, 

 having the formulae :— 



H 



