THE SPACE RELATIONS OF ATOMS. 143 



acids, figs. 7 and 8, Plate II.), although theoretically the 

 tetrahedra should intersect to show the rapprochement of 

 their centres. It will be observed that the R groups 

 occupy one plane with the carbon atoms — a result fore- 

 seen by the tetrahedron-hypothesis (1874), long before it 

 was proved (1882) by the reasoning already set forth. It 

 is evident also that the tetrahedral configurations indicate 

 the existence of the " cis-trans " isomers, which, without re- 

 course to the models, may be indicated by the formulae : — 



R'CR" R'CR" 



II and || 



R'"CR iv R iv CR" 



The same isomerism will exist for CR'R" CR'R". 



The character of this isomerism will be entirely different 

 from that due to the presence of an asymmetric carbon. 

 There is neither dissymmetry nor enantiomorphism in 

 structure, and we shall expect therefore neither opposite 

 optical activity nor hemihedry, and in fact we find none. 



It has been made a reproach to stereochemistry that the 

 same theory which explains the purely physical isomerism 

 of the substances CR'R"R"'R 1V is used to account for the 

 pronounced chemical differences which characterise the 

 isomers C 2 R'R"R"'R 1V . But the new isomers lack the 

 equality of molecular dimensions formerly observed ; the 

 theory is therefore perfectly consistent in demanding that 

 they should differ in chemical as well as in physical pro- 

 perties. And Van't Hoff is now able to enumerate thirty- 

 six cases of isomerism characterised by differences of this 

 kind among bodies of the formula C 2 R'R"R'"R' V . 



Van't Hoff was able at the outset to determine in certain 

 cases which of the two formulae belonged to each isomer, 

 and since then Wislicenus has successfully attacked this 

 problem in other cases. To this end these chemists con- 

 sidered in the first place the mechanism of the addition of 

 atoms to a molecule, and in the second place the mutual 

 influence of the indirectly connected groups. 



In considering the addition of atoms to a molecule they 

 relied on the principle of " least disturbance " ; the structure 

 of the molecule must be as little altered as possible. We 



