﻿Light Absorption and Fluorescence. 



233 



Again, every infra-red band is accounted for, and also the 

 greater symmetry of the molecule is shown by the fact that 

 out of the 14 infra-red bands 11 give rise to pairs of 

 absorption lines. 



The frequency of the central line 3869 = 15 x 258 almost 

 exactly, and 14x258 = 3612 should be the central line of 

 the fluorescence. Only 4 fluorescence bands were observed 

 by Dickson, and the frequencies of these should arrange 

 themselves symmetrically round this as centre. The four 

 frequencies when corrected for solvents are 3504, 3584, 3665, 

 and 3744, and these obviously can be arranged round 3624 

 as centre as shown in Table VII. 



Table VII. — -^-Xylene fluorescence (Dickson). 



o Xin 

 Angstroms. 



1/X. 



1/X + 12. 



>. 



2865 

 2801 

 2739 



2681 



3492 

 3572 

 3653 

 3732 



3504 

 3584 

 3665 

 3744 



120 (117) 



40 (39) 



41 (39) 

 120 (117) 



Now, 3624= 14 x 258*8, and the fundamental interval is 

 therefore very near that of the absorption band system (258). 

 The small number of the fluorescence bands makes it im- 

 possible to arrive at greater accuracy. 



The absorption lines and fluorescence maxima can also be 

 arranged in the same way for o-xylene and ??i-xylene, and 

 they are shown in Tables VIII., IX., X., and XL In these 

 tables certain lines are marked A or F. Those marked A 

 are the lines which Hartley considered to be the heads of 

 the band groups, while those marked F give the same values 

 of v. as appear in the fluorescence spectrum. The wave- 

 lengths of the lines are taken from Mies' paper *. 



The frequency of the central line 3909 = 13 x 300' 7. The 

 next multiple is 12 x 300*7 = 3608, which maybe taken as 

 the centre of the fluorescence bands. In Table IX. the 

 frequencies of the fluorescence bands are increased by 13 

 units to correct for the effect of the solvent. 



* Zeit. wiss. Phot. viii. p. 287 (1910). 



