﻿Liyht Absorption and fluorescence. 22 ( 3 



It will be seen that in the cases of the compounds noted 

 below there are included many pairs of absorption lines 

 which correspond to infra-red bands that have not been 

 observed. In the present state of our knowledge it is im- 

 possible to say whether these infra-red bands really do exist, 

 or whether the absorption line pairs are due to the combina- 

 tion of the molecular rotational frequencies. 



When the substitution products of benzene are considered 

 some difficulty tends to arise, owing to the fact that the 

 molecule becomes un symmetrical. There is no doubt that 

 the simplicity of the structure of the absorption spectrum of 

 benzene is due to the symmetry of the molecule. The absorp- 

 tion spectrum of an alcoholic solution of benzene shows clear 

 evidence of the symmetry. In toluene and its homologues 

 much of this symmetry disappears, and therefore it cannot 

 be expected that the absorption of the vapours of these sub- 

 stances should be so symmetrical. On the other hand, Miss 

 Ewbank and I * showed that in the case of the disubstituted 

 derivatives of benzene the para isomer is always the most 

 symmetrical of the three, and this is well shown in the case 

 of the three xylenes. As will be seen below, ^-xylene shows 

 remarkable evidence of symmetry, while in toluene, o-xylene, 

 and ??i-xylene, the want of symmetry is evidenced by there 

 being fewer absorption lines on the ultra-violet side of the 

 central line. 



Toluene. 



The ultra-violet absorption of toluene vapour has been 

 examined by Hartley t, Grebe J, and others. Hartley stated 

 that the absorption lines resolved themselves into groups 

 with heads at X=2670, 2633, 2605, 2587, 2530, 2471, and 

 2419. Grebe arranged the lines into a number of series 

 with constant differences. The most accurate measurements 

 of the absorption of toluene vapour have been published by 

 Cremer §, who also arranged the lines in series of constant 

 frequency differences. 



In Table IV. are given Cremer's measurements of the 

 toluene absorption lines, arranged in reference to the line at 

 X = 2471 taken as centre. 



Again, in this case there is an exceedingly good agreement 

 between the calculated and observed values of the infra- red 

 bands, the latter being those given by Coblentz, whose paper 

 is quoted as authority throughout. The lettering of the 



* Trans. Chem. Soc. lxxxvii. p. 1347 (1906). 

 t Phil. Trans, ccviii. A. p. 47o (1008). 

 X Zeit. wiss. Phot. iii. p. 376 (1905). 

 § Zeit. wiss. Phot. x. p. p. 349 (1912). 



