Forces between Atoms and Chemical Affinity. 773 



place, the result of the union of the ammonia and hydro- 

 chloric acid could not be correctly represented by the formula 

 NH 3 — HG1. 



As Professor Baker has shown, the union of ammonia and 

 hydrochloric acid does not take place if the gases are dried 

 with extreme care. We can, I think, see a reason for this, if 

 we regard the forces which primarily bring the molecules of 

 the ammonia and hydrochloric acid together as due to electro- 

 static doublets in the molecule. These forces are essentially 

 polar in their nature and depend to a very large extent on 

 the angle between the axes of the two doublets when the 

 molecules come close together ; the attraction is large only 

 when the angle between these axes is small. Thus, when the 

 molecules of the two gases come into collision, it will be only 

 in a very small fraction of the collisions that the circum- 

 stances are favourable to the union of the molecules. If, 

 however, there are nuclei present, such as minute drops of 

 water, then some of the molecules of the gases will, as it 

 were, be anchored round the nuclei : thus a molecule of NH 3 

 and one of HC1 will be kept close together for a considerable 

 time ; this will enable the molecules to arrange themselves 

 under the forces they exert on each other, and they will 

 tend to set so that the axes of the doublets come nearly into 

 line ; the molecules in this position will exert the maximum 

 attraction on each other, and thus be in the condition most 

 favourable for combination. 



A molecule of gas with a large electrostatic moment may 

 itself promote combination between two gases in whose mole- 

 cules there is no intra-molecular ionization. Let us consider 

 the effect of a molecule of this kind on the molecules of the 

 gas round it. The intense electric field round this molecule 

 will drag towards it the molecules around it, so that it will 

 act as a nucleus round which the molecules of the other 

 gases condense. This nucleus will thus bring these molecules 

 closer together than they otherwise would be, and if, like 

 chlorine and hydrogen, they can combine, the presence of the 

 nucleus will assist combination. It seems possible that part 

 of the effect of water- vapour may arise from an action of 

 this kind, the interacting molecules crowding together so as 

 to get into the strongest part of the electric field round the 

 water molecule, and thus getting into positions which are 

 favourable for chemical combination. 



In some cases the product of the chemical action will be 

 active molecules with large moments : this will happen in 

 the case of hydrogen and oxygen, and again with hydrogen 

 and chlorine, as the molecules of water and hydrochloric 



Phil. Mag. S. 6. Vol. 27. No. 101. May 1914. " 3 F 



