﻿224: Prof. E. C. C. Baly on 



evidence itself only as a narrow absorption line, and will 



probably escape detection owing to the comparatively large 



width of slit necessary in infra-red work. The result will be 



that the band will appear to be double, each portion being broad 



since v T is the average rotational frequency of the molecules. 



Bjerrum further pointed out that on the energy quantum 



theory the rotational frequencies must have well defined 



lift 

 values given by the formula v,-= 2 p where I islthe moment 



of inertia, h the Planck constant, and n a whole number. 



As a result of this, an absorption band in the short-wave 

 region of the infra-red should consist of a series of maxima 

 symmetrically distributed about a central line of frequency v. 

 Each pair of maxima will correspond to a definite rotational 

 frequency of the molecules. Now Fraulein von Bahr has 

 made very accurate measurements of the absorption band of 

 water vapour at 6*26 /*, and found clear evidence of these 

 pairs of maxima*. From these she calculated the wave- 

 lengths corresponding to the rotational frequencies, and 

 showed an excellent agreement with the absorption bands 

 as observed by Eubens and v. AVartenberg. Eucken | has 

 shown that a still better agreement is obtained on the basis 

 of there being two degrees of freedom possessed by the 

 water molecule. The experimental evidence therefore most 

 strongly supports Bjerrum's theory. 



Now it is well known that the absorption bands of many 

 organic compounds in the ultra-violet can be resolved into 

 groups of fine lines, each group very frequently possessing 

 a well marked head. It has occurred to me, if the bands in 

 the short-wave infra-red region are due to atomic vibrations 

 and those in the ultra-violet are due to electronic vibrations, 

 that it should be possible to combine the two in the same 

 way as has been done by Bjerrum. That is to say, if v be 

 the characteristic vibration frequency in the ukra-violet, 

 then we should find pairs of absorption lines with frequencies 

 equal to v±_v x , where v x stands for the frequencies of the 

 short-wave infra-red bands. It is evident that the applica- 

 tion of the theory can be tested on ultra-violet absorption 

 hands with far (greater accuracy than in the case of the 

 infra-red bands for two reasons. Firstly, the measurement 

 of the ultra-violet bands is much more accurate ; and 

 secondly, we know far more of the absorption bands in the 

 short-wave region of the infra-red than of those in the lono-- 

 wave region. 



* Phil. Mag. xxviii. p. 71 (1914). 



t Deutsch. Phys. Ges. f Verh. xv. p. 1159 (1913). 



