﻿Absorption Spect?>a of Organic Substances. 193 



The model of the atom from which the results are obtained 

 is that in Avhich the valency electrons are supposed to move 

 in circular orbits round a central nucleus; when the valencies 

 become saturated, the corresponding electrons are supposed 

 to be withdrawn out of their original orbits, perhaps into one 

 of greater radius (about the line joining the nuclei) the 

 periods in which are much greater than in the original. On 

 this hypothesis, it is clear that when an atom is unsaturated 

 as to one primary valency like the carbons in ethylene, there 

 will remain one electron only in the outer orbit. If, now, 

 a disturbing force such as light passes through the system, 

 the electrons will vibrate about their orbits, and, from what 

 has been said above, it appears that the periods of primary 

 electrons of some of the commoner atoms happen to lie in or 

 near the region examined in the ordinary instruments. The 

 case when there is one such seat of disturbance has been fully 

 dealt with by Drude 1 and others ; here cases where several 

 such " vibration centres " occur in the molecule will be dealt 

 with. 



Since an electron moving in an orbit is a current, the 

 vibrations of one will affect those of another in accordance 

 with the laws of electrodynamics (see Appendix) ; the 

 problem is therefore reduced to compounding vibrations by 

 means of the mutual induction method. 



It is not of course claimed that the whole of the mathe- 

 matical treatment is original : it is, however, claimed that 

 most of the cases worked out are new, and that this method 

 has not been applied to the problem before. 



There are, however, certain sources of uncertainty : in the 

 first place the work is so complicated that only the simplest 

 cases, i. e. those in which there is a certain amount of sym- 

 metry, are profitably discussed ; we assume, therefore, that 

 slight changes not involving the introduction of a fresh 

 vibration centre will not make any difference to the type of 

 spectrum. 



Again, there are on our theory several rings of electrons 

 in a molecule, and these will all have some influence on 

 one another. But while every electron in the molecule 

 vibrates in a complex manner compounded of all possible 

 periods, the most marked periods will be those which 

 most closely approach the natural period of the electron in 

 question. The effect of the other rings may explain the tine 

 lines into which broad bands occasionally split under favour- 

 able conditions. In connexion with this it must be observed 

 that the convenient abbreviation that " the band at (say) 

 X 300 is due to oxygen in the carbonyl group" does not 



Phil Mag. S. 6. Vol. 29. No. 169. Jan. 1915. 



