496 SCIENCE PROGRESS. 



In I. like groups are on the same side, in II. on the 

 opposite sides of the molecule ("cis-trans" isomerism, also 

 called, in a special sense, "geometrical " isomerism). 



R' R' 



Formula I. yields R" C C R" (a) 



R" R" 



R R" 



and formula II. yields R"' C C R'" (ft) 



R" R' 



In a each CR'R'R"' group is the mirrored image of the 

 other, and this will be the case whether the R"' groups are 

 attached above or below the plane of the paper. In ft each 

 CR'R'R" group can be superposed on the other, hence each 

 group possesses activity of the same sign and the molecule 

 is doubly active ; but if the R'" groups instead of attaching 

 themselves (say) above the plane of the paper, attach them- 

 selves below, we have another body formed. In this, also, 

 the CR'R'R" groups are superposable, and the molecule is 

 doubly active ; but its activity is directly opposed to that of 

 its isomer, because the order of the R'R"R'" groups has been 

 transposed. And since the R'" groups will act as often on 

 one side of the molecule as on the other, the isomers will be 

 produced in equal quantities. 



Such a case is presented by the formation of mesotar- 

 taric acid on oxidising maleic acid, and of racemic acid on 

 applying the same oxidising agents to fumaric acid. Hence 

 the observed results compel us to accept a plane arrange- 

 ment in this case, and we may fairly assume that the same 

 arrangement holds for other derivatives of ethylene. In 

 complete accordance with this we find these substances 

 uniformly inactive even when possessing the formula 

 A R R" R"' R iv (A = C 2 ). An exception was announced in 

 the form of an active citraconic acid CH ■ COOH • C ' CH 3 • 

 COOH, but Le Bel now finds that the active substance he 

 obtained by the action of microbes on the inactive acid 

 was methyl-malic acid. 



Again there is reason to believe that, as a rule, benzene 

 derivatives possess plane molecules. Chemically, there is 



