520 
DR. WALTER NOEL HARTLEY ON THE ABSORPTION SPECTRA 
lowering of the temperature by 1 0, 2 C., and increasing the pressure by 23*5 mm. 
The quantity of benzene was practically the same in both cases : in (2) the weight 
found was 0'0179 grm., and in (3) that calculated from the vapour pressure was 
0-0175 grm. The difference is seen in (3), where the increased general absorption 
cuts off Band YI. entirely. It also widens the absorption bands, so that the heads 
are measurable at a point from 1 to 3 Angstrom Units extended towards the rays of 
shorter wave-lengths. 
A solution band is thus seen to be the aggregate effect of the absorption of rays 
caused by a group of vapour bands, which is modified by the greater density, and 
consequently greater dispersion, of the liquid medium in which the substance is 
dissolved, and it is shifted towards the red in accordance with Ktjndt’s rule 
(A. Kundt, ‘ Pogg. Ann.,’ Jubelband, 1874, p. 620). The slightly different positions 
of the four vapour spectra (see map, p. 484), their similarity in constitution, and the 
manner in which they overlap, afford an explanation of the reason for the spectrum of 
benzene being composed of six principal groups of vapour bands and six corresponding 
solution bands. The constitution and properties of the vapour spectra account also 
for four of the solution bands being similarly constituted, and of greater width and 
intensity than the others, with approximately the same persistency or coefficient of 
extinction ; they also account for the isolated narrow bands at each end of the 
spectrum. It is evident, on examining the map (on p. 484) and comparing it with the 
photographs (figs. 1 and 4), that where the four vapour spectra overlap to the greatest 
extent, the four most regularly constituted and intense groups of bands are to be 
found, and the four most intense solution bands naturally occur. At either end, 
where the overlapping is less complete or does not occur, the first and sixth solution 
bands are situated, and likewise the narrow and unimportant groups and isolated 
bands. Under certain conditions, such as diminished pressure or increased tempera¬ 
ture, other feeble bands of the vapour appear distinctly at either end of the spectrum, 
and with reduction of pressure they become strong bands at the less refrangible end. 
A further noticeable feature in each of the four vapour spectra is that the bands are 
generally grouped together in threes and fours, also occasionally in fives, and as this 
affects the constitution of the solution bands y and particularly their width, it possibly 
may account for their number being six. 
The Homologues of Benzene. 
It lias been shown that a displacement of the general absorption towards the less 
refrangible rays in solution spectra is caused when the molecule is loaded by the 
replacement of one or more H atoms in benzene by elements of greater atomic weight, 
or by radicals or complex groupings such as Cl, Br, CH,, C 2 H 5 , or CH-CH 3 -CH 3 , 
the displacement being the greater the heavier the load. As regards the selective 
absorption, there is a complete divergence from the spectrum of benzene, inasmuch as 
