494 SCIENCE PROGRESS. 



hydrogen atoms is at the corners of two parallel equilateral 

 triangles, as in the prism formula for benzene ; on sub- 

 stituting two atoms of chlorine for two atoms of hydrogen, 

 we should have, then, three isomers as in the case of ben- 

 zene ; but there are only two. More complicated arrange- 

 ments would give a still wider divergence. Therefore no 

 fixed arrangement of the hydrogen atoms exists, but they 

 are constantly changing places. 



This argument, however, will not hold ; for as Le Bel 

 has himself urged, the number of positions of equilibrium 

 of a molecule depends on the particular properties of the 

 elements composing it, and not at all on the positions which 

 the replaced atoms occupied before replacement. In other 

 words, there are two positions of equilibrium for the chlorine 

 atoms in C 2 H 4 Cl 2 , and when this compound is formed from 

 C 2 H 4 H' H", the atoms will re-arrange themselves until the 

 chlorine atoms reach one or other of their two possible 

 positions, whether these were previously occupied by the 

 displaced hydrogen atoms H' H" or not. There is then no 

 theoretical ground for opposing Le Bel's earlier view that 

 A may be any radical, simple or complex. 



Indeed it seems probable that we must look in the 

 opposite direction for exceptions of the first class. It is 

 when the radicals R' R", etc., are small and light, that 

 we should expect the mobility of the chaotic state. In 

 particular, the hydrogen atom would be suspected of de- 

 stroying the stability necessary for asymmetry. A notable 

 instance is afforded by the tri-substituted ammonium chloride 

 (R R" R"' R iv R v = Me Et Propyl H and CI), which Le 

 Bel found to be inactive, while on replacing the last hy- 

 drogen by a compound radical (isobutyl), activity resulted. 

 Again, the inactivity of bromnitro-ethane, CHBrN0 2 CH 3 

 has suggested a desmotropic action of the hydrogen atom 

 as an explanation. This cannot be accepted in face of the 

 fact that such substances as methyl-ethyl-carbinol, CH • CH 3 ' 

 C 2 H 5 • OH, propylene-glycol, CH ■ CH 3 • CH 2 OH ■ OH, and 

 lactic acid, CH • CH 3 • OH ■ COOH, are active. At the same 

 time simplicity of structure certainly favours mobility, as 

 witnessed by the fact that asymmetry has never been 



