lO INFRA-RED ABSORPTION SPECTRA. 



process, is accompanied by a change in the physical molecule that a 

 change occurs in the absorption spectrum ; and, vice versa, each change 

 in the character of the absorption spectrum is connected with a change 

 m the physical molecule. Hence, so long as the physical molecule has 

 not changed, Kundt's law, of shifting toward the red, holds. In a 

 solid state, i. e., at a low temperature, the physical molecule is more 

 complex. As a test of this last question we have the observations of 

 Wiedemann^ on iodine solutions. Iodine in CS2 is violet, similar to the 

 gaseous condition, while in alcohol it is brown, like melted iodine. The 

 latter is the more complicated. Hence, if this assumption be true, we 

 would expect the violet carbondisulphide solution to turn brown on 

 cooling. This is the case. He also observed that cold brown solutions 

 of iodine in stearic and oleic acid became violet on heating to 80°. 



The absorption spectra of the alkaloids has been studied by Hartley^ 

 and others, and a remarkable similarity has been observed in their 

 absorption spectra. After examining 30 alkaloids they think that, " as 

 a general rule, those which agree closely in structure give similar 

 absorption spectra, while those which differ in essential points of struc- 

 ture give dissimilar spectra. Most of these compounds have high 

 molecular weight, and changes may be effected in their molecules with- 

 out alteration of their spectra, which, in substances of lower molecular 

 weight, would be attended by wide differences." Hence the alkaloids 

 differ only in details of structure. This effect will be noticed, in the 

 present work, for petroleum distillates. 



The effects of dilution, of temperature, of acids, and of different solv- 

 ents upon the absorption spectra of solutions of didymium and erbium 

 salts have been investigated by Liveing.^ He found that the spectra of 

 the different salts of the same metal in a dilute condition are identical. 

 Ostwald interprets this by saying that the spectrum, common to all the 

 salts of the same metal, is due to the metallic ions. All the tests applied 

 by Iviveing contradict this assumption. He explains his observations 

 by assuming that, in solution, the molecules are ruptured by collisions, 

 but immediately recombine. Increased temperature and concentration 

 mean more frequent encounters amongst the molecules and more 

 frequent ruptures, which are counterbalanced by the more frequent 

 encounters of the parts. These effects will compensate each other and 

 leave the average number of absorbing parts of molecules constant, 

 under changes of temperature or concentration as observed. In other 



'Wiedemann : Ann. der Physik (3), 41, p. 299, 1890. 



'^Hartley : Phil. Trans., Part II, 47, p. 691, 18S5 ; Hartley & Dobbie : Phil. 

 Trans., 77, p. 846; Dobbie & Lawder : Chem. Soc. Jour., 83 and 84, pp. 605 and 

 626, 1903. 



^Liveing : Trans. Cambridge Phil. Soc. xviii, p. 298, 1900. 



