238 



R; 



in the molecule, i.e. of the necessity of making use of stereometrical, 

 instead of plane structural formulae for the representation of 

 molecular composition and configuration. 



As Van 't Hoff's method of demonstration appears more 

 suitable for its purpose than Le Bel's, we shall chiefly use the 

 former in developing the following reasonings. 



According to Van 't Hof f, the four valencies of the carbon-atom 

 may be considered as forces issuing from the carbon-atom, and 

 like "vectorial" quantities, determined by magnitude and direction 

 in space. As to their size, we dare not hazard a guess, as nothing 

 certain is known about it, and further it is highly probable that it 

 varies considerably with the special nature of the groups connected 



with the carbon-atom. As to 

 the direction, however, Van 

 't Hoff makes the simple 

 supposition, that in compounds 

 in which the four carbon- 

 valencies are saturated by 

 four identical substitutes, the 

 four forces are directed like 

 the lines joining the centre of 

 a regular tetrahedron with its 

 corners. Thus the configu- 

 ration of compounds such as 

 methane: CH, tetra-methylme- 

 thane: C(CH 3 ), carbon-tetra-iodide: C/ 4 , etc., may be represented 

 by a scheme such as in fig. 162. This arrangement of atoms possesses 

 of course a high degree of symmetry: if the groups R t be sup- 

 posed to behave as substitutes having spherical symmetry, the 

 whole arrangement possesses at least the symmetry of the group 

 T D . Indeed, the supposition that the groups R t always behave in 

 this respect, as if they had the greatest possible (spherical) sym- 

 metry, except in the case when they are non-superposable with 

 their mirror-images, is of vital interest for the facts to be discussed 

 in the following pages. 



If, for instance, the group ^ were to be considered as fully asym- 

 metrical, it might happen that the molecule as a whole had no planes 

 of symmetry at all, and only axial symmetry. In that case, if no 

 axes of the second order were present, the arrangement would cer- 

 tainly differ from its mirror-image, and two enantiomorphously 



Fig. 162. 



