243 



Pasteur's first postulation, namely: the general and constant 

 connection between molecular dissymmetry and optical activity, 

 has been firmly established by experiment ; and also, that the doctrine 

 of the so-called "asymmetric plurivalent atoms", as proposed by 

 Van'tHoff and Le Bel really gives in most cases a sufficient and 

 rational explanation of the significance of this molecular dissym- 

 metry. In general features this is true: nevertheless there remain 

 some difficulties, which can best be elucidated if we go somewhat 

 further into the essentials of Van 't Hoff's idea. 



Properly speaking, the doctrine of the asymmetric carbon-atom 

 deals with two factors at the same time, which are superposed the 

 one upon the other, without it being quite certain that they must 

 necessarily always be mutually connected. For Van 't Hoff's 

 conception not only contains the idea of the dissymmetrical arrange- 

 ment of atoms or radicals in the molecules; but it lays stress also 

 on the chemical differences which, according to these views, must 

 necessarily co-exist between the substitutes, which are dissymme- 

 trically placed round the asymmetric atom under consideration. 



This last view is, however, by no means justified: in the preceding 

 chapters we have often met with geometrical complexes and arran- 

 gements in space of identical structural units, which nevertheless 

 represented stereometrical configurations differing from their mirror- 

 images. It turned out that such arrangements, built up of identical 

 units, could still appear in enantiomorphously related forms. The 

 condition of the "dissymmetrical configuration" in Pasteur's 

 meaning of the word, including the possibility of the occurrence of 

 right-, and left-handed isomerides, is, therefore, by no means exclusi- 

 vely dependent on a difference between the constituting unities of 

 the complex considered. Even if they are identical, the molecule as 

 a whole may possibly manifest such a non-superposable symmetry 

 of its configuration. 



In the same way we may put the question: can different atoms 

 or radicals eventually be arranged in such a way round a plurivalent 

 atom, as to form a higher symmetrical complex not differing from 

 its mirror-image? The answer must certainly be that they can: in 

 such a case we should have to imagine that the four different sub- 

 stitutes /?,, R', R", R'" placed round the central atom, notwith- 

 standing their difference in chemical nature, were placed at the 

 corners of a regular tetrahedron or of another symmetrical figure, 

 no appreciable distortion being caused for some reason or other. 



