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XXI. A Study of the Absorption Spectra of Organic Sub- 

 stances in the Light of the Electron Theory. By X. P. 

 K. J. O'X. McCleland, B.A., Pembroke College, Cam- 

 bridge *. 



[X.B. — The small numbers refer to the table of references given at 

 the end.] 



THE large number of experimental results now available 

 has made it possible to trace a qualitative, and quite 

 empirical relation between the constitutions of organic 

 substances and their absorption spectra. Thus, it has been 

 found that certain groupings of atoms are likely to give 

 rise to bands in particular regions of the spectrum, but all 

 theories which have hitherto been proposed to explain this 

 fact appear to have postulated a characteristic type of vibra- 

 tion within each kind of group. Thus in the isorropesis 

 theory of Baly a vibration between the forms 



— Oils. C=0 -•* — OH=C— OH 

 I I 



was suggested £ n the ketones, and between 



0=0 C— 



I - II I 



0=0 0—0 



in the diketones, and so on. The only generally accepted 

 law was that selective absorption in the ordinarily examined 

 region | originates in the unsaturated condition of atoms, 

 and that the accumulation of these unsaturated valencies 

 shifts the absorption towards the red end of the spectrum. 



In this paper a theory is proposed which, starting from 

 the atom, builds up the spectrum in a perfectly general way 

 and makes it possible to predict the positions of absorption 

 bands with reasonable accuracy from the constitution, given 

 certain fundamental constants. At present, these have only 

 been obtained in a few cases owing to the lack of necessary 

 data, and the author is unable to obtain these for want of a 

 spectroscope. 



* Communicated by Sir J. J. Thomson, O.M., F.R.S. 



The author being under orders for active service has been obliged to 

 leave some points in this paper unfinished, for example an extended 

 table of numerical results might have been given. It is hoped that 

 these deficiencies may be made up in the future. 



t Until recently spectroscopes in general use covered the region 

 X 600 — 230 (nn). Stark and his collaborators have lately published 

 results down to X 180. (See below.) 



