Absorption and Fluorescence. 423 



and then on further dilution it shifts towards the shorter 

 wave-lengths, until it finally reaches the true value for the 

 vapour. 



One further example may be given, namely, that of aniline, 

 which is interesting from the fact that it shows two absorp- 

 tion-band groups in alcoholic solution. The central wave- 

 numbers of these two bands have been measured at very 

 great dilution (V = 84,000) and were found to be 3496'7 

 and 4255 respectively. Now these two wave-numbers must 

 be even multiples of the fundamental infra-red wave-numbei\ 

 and therefore we have 



3496-7 _ 4255 _ 

 x y " ■? 



where v x is the true fundamental infra-red frequency of 

 aniline and x and y are whole numbers. The only solution 

 is given by a? =23 and y = 28, whence the true values of 

 v x are found to be 152*03 and 151*97 respectively. Now 

 Purvis * has observed the less refrangible absorption band 

 of aniline vapour, and from his measurements it is clear that 

 the central wave-number is l/\ = 3496*1. As this value is 

 obviously more accurate than that obtained from the solu- 

 tion, we may take the true value of v x for aniline to be 

 3496*1/23 = 152, which is a mean of the two solution values. 

 It follows, therefore, that aniline vapour should show a 

 very strong absorption band at 1/152 = 6*58 /a, or at the first 

 multiple of this, 3*29 /jl. The absorption of a liquid film of 

 aniline has been observed by Coblentz f, and in comparing 

 the absorption spectra of vapour and liquid it must be 

 remembered that in the latter the bands will have slightly 

 longer wave-lengths. The value obtained by Coblentz was 

 3*34 fi instead of 3*29 /jl. 



These results are of some importance, for, in the first place, 

 they show that the relationship between the infra-red and 

 ultra-violet absorption bands pointed out in the previous 

 papers, namely that the central wave-numbers of the latter 

 are whole multiples of a fundamental wave-number in the 

 infra-red, holds good in solution. In the second place,, 

 they would seem to offer an explanation of the well-known 

 fact that in general there is a shift in the position of an 

 absorption band when the substance is dissolved in a solvent.. 

 Since the central wave-number is a whole multiple of a 

 fundamental infra-red wave-number, the shift must be due 

 to the fact that- the fundamental wave-number changes on 



* Trans. Chem. Soc. xcvii. p. 1546 (1910;. 

 t Loc. cit. 



