229 



R; 



A 



course, an explanation of this kind <.! phenomena must involve the 

 primary supposition of a stereometrical arrangement of the atoms 

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

 HIM <ad 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's in developing the following reasonings. 



According to Van 't Hoff, 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 configuration of 

 compounds such as methane: 

 CH t , tetra-methyl-methane: 

 C(CH 3 ) t , carbon-tetra-iodide: 

 CJ 4 , etc., may be represented 

 by a scheme such as in fig. 160. This arrangement of atoms possesses 

 of course a high degree of symmetry : if the groups R l be supposed 

 to behave as substitutes having spherical symmetry, the whole 

 arrangement possesses at least tht symmetry of the group T D . 

 Indeed, the supposition that the groups R l always behave in this 

 respect as if they had the greatest possible symmetry, 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 R l 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- 



Fig. 1 60. 





