231 



not jK>ssess any symmetry-property of the second order whatever. 



Van 't Hof f calls a car- 

 bon-atom, the four valen- 

 cies of which are saturated 

 by four different univalent 

 atoms or radicals, an asym- 

 metric carbon-atom. He was 

 able to indicate the presence 

 of at least one such asym- 

 metric carbon-atom in all 

 compounds which in the 

 liquid or gaseous state ex- 

 hibit, the power of devia- 

 ting the plane of vibration of the polarised light. Since then, this 

 hypothesis has proved valid in a great number of cases; and it was 

 by this conception of the plurivalent asymmetric atoms, that a rational 

 definition of the circumstances under which Pasteur's "molecular 

 dissymmetry" was found, seemed to be given now, thus enabling 

 the science of stereochemistry to be founded, as we know it to-day. 



It should be 

 remarked that by 

 substituting for 

 R! other radicals 

 R', R", R"', etc., 

 a gradual distor- 

 tion of the mole- 

 cule takes place: 

 not only are the 

 distances of the radicals R lt R', R", R'", etc. from the central carbon- 

 atom undoubtedly different, but as a consequence of the mutual at- 

 tractions and repulsions of the substitutes, the forces issuing from 

 the carbon-atom will certainly deviate from those supposed in the 

 case of fig. 160, provided always that we accept the rather rough 

 mechanical image proposed by Van 't Hoff. In the case of a com- 

 pound: CR^'WR'", the "regular" tetrahedral arrangement of 

 fig. 160 will have quite disappeared ; it will be replaced by a wholly 

 irregular one. This must be firmly kept in mind, even though it should 

 appear equally true that the two imaginary arrangements a and b 

 in fig. 162 would again seem to be enantiomorphously related, if 

 we had the power to replace the radicals R by different groups without 



Fig. 162. 



