Electric Origin of Molecular Attraction. 633 



paper just referred to (p. 45), it u means that the mutual 

 energy of two molecules of this type divided by the number 

 of equivalents in each can be obtained by regarding each 

 equivalent as a separate attracting entity." For if each 

 pair of electrons forming a chemical bond attracts each other 

 one, then in Clausius's equation of the virial if, neglecting 

 for the present purpose external pressure, we write 



i~Nmv 2 =i.it%3a 2 r/r\ 



where the double summation %% is to be effected for all the 

 N molecules, we have really to sum for the electric doublets 

 which produce the molecular attraction. In the first place, 

 then, we have to evaluate S3aV/r 4 , where r represents the 

 distance of any one definite molecule from any other, and 

 the summation is to be effected for all molecules within a 

 distance L. Now if there are n doublets in each molecule 

 and a, is the value of a appropriate to a doublet, this would 

 take the form n^Xofir/r*, if each doublet were associated with 

 a molecule entirely its own. But as there are n doublets in 

 each molecule, it is clear that in general the parameter a 

 cannot be equal to net. But there is an important exception 

 to this, namely, when the axes of the doublets are all directed 

 the same way, so that their moments are simply added 

 together and then a = na. We shall see in section 5 that 

 complex molecules show a tendency towards this state of 

 identical direction in the doublets which they contain. But 

 the case of the simpler types of binary compounds is one 

 where considerations of symmetry do not favour the hypo- 

 thesis of similarly directed doublets in the molecule. For 

 example, the structure of CaCl 2 would be best represented by 

 the formula bCl#Ca£Clb, where the two doublets are oppo- 

 sitely directed. Jn the case of SnCl 4 we should expect the 

 four doublets to be pointing from the centre to the corners 

 of a regular tetrahedron. The collision of molecules carrying 

 doublets directed in such ways as these can be regarded in 

 the following manner. The circumstance chiefly directing 

 the occurrence of attraction between two molecules is that 

 the doublet in one which is nearest to a doublet in the other 

 should have its axis in nearly the same direction as that of 

 the latter. It is true that there are w 2 ways of arranging 

 two molecules so that they may have a pair of doublets in 

 the most favourable position for attraction. But out of every 

 n chances which the n doublets give a molecule of being 

 attracted by another only one eventuates in attraction, be- 

 cause it happens to be the strongest and ultimately leads to 



