THE SPACE RELATIONS OF ATOMS. 135 



tical, giving only one C(R') 2 R"R'" ; whereas a plane formula 

 must have at least two configurations for this substance, 

 representing two isomers : — 



R R' 



R"_C_R'" and R"— C— R' 



R' R" 



where only one actually exists. For a similar reason pre- 

 ference must be given to the tetrahedral grouping for 

 C(R') 2 (R") 2 ; for C(R') 3 R" and for C(R') 4 both the solid 

 and the plane formulae accord with the facts in representing 

 only one configuration; but for these, too, Van't Hoff pre- 

 fers the former. 



" My fundamental idea was the tetrahedral grouping, 

 that is to say, some force — cause unknown — proceeding 

 from the carbon atom and tending to drive the groups 

 united to carbon as far away from one another as possible, 

 that is, to bring them into the tetrahedral position. Al- 

 though it did not follow that the tetrahedron must be 

 regular, because the mutual action of the different groups 

 may vary it somewhat, yet the tendency to form the regular 

 tetrahedron remained, and in the case of identity among the 

 groups, as in CH 4 , the tendency was realised" (A tome im 

 Raume, p. 68). 



As to this, Le Bel holds that it depends entirely on the 

 relation between the attraction of C for H on the one hand, 

 and the repulsion between H and H on the other. He as- 

 sumes x that atoms attract one another up to a certain point, 

 but on closer approximation repel one another. We have 

 then for CR 4 two positions of equilibrium of R 4 about C. 

 A symmetrical (regular tetrahedral) arrangement will 

 result if the repulsive zones of R occupy the whole 

 surface of the zone C or a greater surface ; but if 

 less, a non-symmetrical arrangement will result. (They 



1 Bull. Sac. Chim. [3], iii., 788. 



