772 Sir J. J. Thomson on the 



If, however, instead of an atom 'we had a molecule in 

 which there was intra-molecular ionization, its moment 

 would be far larger than that of the atom, and it would be 

 attracted by the field of force round AB with much greater 

 intensity. The force might be large enough to bind the 

 molecule to AB, and we should have a system represented 

 by AB — M. Thus molecules like those of water, ammonia, 

 alcohol, sulphur trioxide, which have large electrical moments, 

 might be expected to enter into combinations of this kind, 

 forming complexes of two or more molecules. This is the 

 type of compound called by chemists molecular compounds, 

 and which at one time were often called into requisition to 

 explain the existence of compounds which did not follow the 

 law of constant valency. Of late, however, the idea of 

 molecular compounds seems to have gone out of favour. 



It seems to me that when we take into consideration the 

 strength of the electric field exerted by some molecules, it is 

 very difficult to believe that molecular compounds are not 

 formed, and that they do not play a large part in many 

 chemical reactions. We have seen (p. 770) that union with 

 another molecule promotes intra-molecular ionization ; thus 

 it might happen that the atoms in AB were uncharged when 

 AB was isolated, but when it had attracted another molecule 

 a corpuscle would pass between A and B so that these atoms 

 would get charged. In this way the properties of AB might 

 be entirely changed by union with another molecule. We 

 can, I think, go further than this. May there not be cases 

 in which the attached molecule or molecules act on AB in 

 much the same way as the molecules of a solvent would act 

 upon it in solution, so that the atoms of A and B, in addition 

 to getting charged through the proximity of the molecules 

 attached to AB, might get separated by a kind of intra- 

 molecular dissociation, and then rearrange themselves so as 

 to be in equilibrium with the atom of M, the molecule 

 attached to AB. 



To take the most famous case in connexion with the 

 formation of molecular compounds, that of sal-ammoniac. 

 We may regard this as produced by a molecule of ammonia, 

 which as we have seen has an exceedingly large electrostatic 

 moment, being attracted by the electric field round the 

 molecule of hydrochloric acid, This is only the first stage : 

 may not the HOI, under the action of the molecule of 

 ammonia, not merely have the H atom positively, the CI atom 

 negatively electrified, but the two torn apart from each other ? 

 The four H atoms, and the N and 01 atoms might then 

 regroup themselves so as to be in equilibrium. If this took 



