﻿Light Absorption and Fluorescence. 



231 



infra-red at X=8*48^, an almost exact mean o£ the two 

 bands observed at 8*40 and 8'53 fi. 



It is interesting to note that the central line at l/\ = 4047 

 is very nearly the same as that o£ benzene (4055). Now 

 4047 = 10x405 very nearly, and therefore 9x405 = 3645 

 should again be the central line of the fluorescence maxima. 

 If Dickson's values for the fluorescence of toluene be taken, 

 it is clear that the maximum at 3650 must be the centre of 

 the system. The maxima can be arranged as in Table V., the 

 figures in brackets again showing the corresponding values 

 of Vx obtained in the absorption band. The accuracy of 

 determination of the wave-lengths of fluorescence maxima is 

 not very great, and if the correction for solvent be applied 

 here there appears to be an error of about 9 units in the 

 value of 1/A, for the central line. This is well within the 

 limit of experimental error. 



Table Y. — Toluene fluorescence (Dickson). 



o Xin 

 Angstroms. 



1 



l/X. 





2886 

 2809 

 2740 

 2676 

 2646 

 2622 



3465 

 3561 

 3650 

 3737 

 3779 

 3814 



185(186-5) 



89 (86) 

 



87 (86) 

 129 (131) 

 164 (159) 



Again a very good agreement is shown between the 

 values obtained from the absorption and fluorescence spectra. 



The want of symmetry of the toluene molecule is well 

 shown by the fact that out of the 19 infra-red bands only 5 

 give pairs of absorption lines, the remainder being evidenced 

 only by lines on the red side of the centre. 



p-Xylene. 

 Although the vapour absorption spectrum of p-xylene was 

 investigated by Hartley, more accurate measurements have 

 been published by Mies*, who showed that the fine lines 

 can very readily be arranged in series. He observed in the 

 spectrum a certain number of strong lines which he denoted 

 by A, and also a number of slightly less strong lines which 

 he denoted by B. The A lines and the B lines form two 

 series with constant frequency differences. Then, further, 

 lie found other lines, the frequencies of which differ by 

 definite amounts from the frequencies of the members of the 



* Zeit wist. That. xu. p. 357 (1909). 



