666 



Mr. N. P. McCleland on the 



two atoms. The model adopted is therefore the one suggested 

 by Professor Rutherford. 



It was further supposed that while atoms are unsaturated 

 as to (say) one primary valency, there is still one electron in 

 the outer ring of those revolving round that atom (though 

 the radius of the orbit may be changed); consequently a 

 molecule of ethylene may be represented as in fig. 1. 



Fig. 1. 



-H 



(3 



H 



where 



a, a represent the orbits of electrons uniting C and H. 

 fi represents „ „ „ and C. 



7, 7 represent the orbits of unsaturated electrons *. 



This, of course, represents only the mean position about 

 which the system oscillates. 



There are possibly other rings of electrons round C and H 

 representing higher valencies. Since these would be nearer 

 the central nucleus, they would be more difficult to remove 

 than those in the outer rings, and perhaps for this reason do 

 not enter into chemical combination. 



They do, however, enter into physical chemistry as residual 

 affinity. 



There are some points of interest in connexion with the 

 above which must be mentioned before passing on to the 

 main subject of this paper, namely, an investigation whether 

 the above model is consistent with chemical facts. 



In the first place, the interactions between various groups 

 round an atom will bring the groups into definite positions. 



* When the conditions of equilibrium are written down it is found 

 that there is one degree of freedom, namely, the " scale " of the system 

 or the actual distance between two chosen atoms. Since some elements 

 of the system are fixed, namely the electric charges, the conditions of 

 stability are likely to put limits to this scale ; and the conditions such 

 as temperature, solvent, &c, will limit them still more ; therefore pro- 

 perties such as absorption spectra will depend on the conditions. 



