776 Sir J. J. Thomson on the 



either of the compounds AB or AB 2 . The attachment of the 

 B molecules will get looser and looser as successive molecules 

 of B are added, until finally we reach AB„, in which the 

 work required to detach a B molecule becomes comparable 

 with the energy corresponding to one degree of freedom at 

 the temperature of the compound. This compound AB n will 

 be unstable, or rather will not exist in appreciable quantities, 

 so that the compound AB n _ L will be the one richest in B 

 that can be detected. There will thus be a definite limit to 

 the number of B's that can be bound to the A molecule, 

 and we should expect from the values found for the moments 

 of the electrostatic doublets in the cases where we have the 

 data for calculating them, that this number would not be large. 

 "Water of crystallization is a case of this kind, B representing 

 the water molecule ; in general the number of molecules of 

 water of crystallization is not large. If the molecule A con- 

 tained several atoms, there might be several regions where the 

 molecules B might find a place. If A, for example, contained 

 two atoms, a and /3, and the electric fields round a and /3 

 were equal in intensity, then if the field round M were 

 symmetrical we might have molecules of B attached to 

 a. and /3 as in the scheme 



B-a/3-B. 



More molecules of B might be added if the fields due to 

 a and /3 were strong enough, some to the left, an equal 

 number to the right, so that the B molecules in a compound 

 of this type would be done up into two bunches. If* the 

 molecule A had a more complex structure, there might be 

 more than two different regions in which the B molecules 

 could group themselves, so that the B molecules attached to 

 an A molecule might be divided into more than two groups. 

 We see in this way that the number of water molecules 

 which are able to exist in any one group may be con- 

 siderably less than the number of molecules of the water of 

 crystallization. 



The electrostatic effect of a doublet at a considerable 

 distance from its centre depends only on the moment, — the 

 product of either charge and the distance between the two 

 charges. At close quarters, however, other things may come 

 into consideration ; for example, if we compare the charged 

 atoms in the doublet to two charged spheres, the moment at 

 a considerable distance is independent of the radii of these 

 spheres if the distance between their centres remains constant. 

 Close up to these spheres, however, the electric force will 

 depend upon their radii, and will be greater at the surface 



