526 
DR. WALTER NOEL HARTLEY ON THE ABSORPTION SPECTRA 
unaffected in the meta-xylene. The merging of the narrow bands in the different 
parts of the spectra of the three isomers is evidently connected with the positions in 
the benzene ring where substitution of CH 3 for hydrogen has been effected. 
It is proved that benzene vapour at 100° C. has the same molecular mass as at 
25° C. or 12°7 C., so that at lower temperatures the molecules are not associated 
when in a state of vapour. 
The absorption bands at 100° C. are seen on comparison to be almost identical with 
those at lower temperatures, but at 100° C. there is this difference, that the bands in 
the region about A 2700 are of augmented intensity, with a peculiar sharpness and 
definition which is wholly wanting at lower temperatures, as they then appear feeble 
and ill defined, or do not appear at all. 
The similar groups of bands occurring in benzene and toluene, and the close 
similarity between the spectra of toluene and ethylbenzene, with the further resem¬ 
blance between meta-xylene, toluene, and ethylbenzene, is evidence that the peculiar 
mode of vibration of the benzene nucleus, or ring-structure, is in a great measure 
unaffected by the side chain substitution in these compounds. 
The important influence of the position of the substituted hydrogen in benzene 
upon the number and position of bands in the spectra of the homologues is clearly 
demonstrated. 
The general absorption observed in solution spectra , like that in vapour spectra, is 
also apparently caused by the translatory motion of the molecules. The selective 
absorption, consisting of the coalescence of several narrow bands, is regarded as due 
to the intra-molecular motions; motions of the atoms, or atom-complexes, within the 
molecules, upon which, if the solution be concentrated, a general absorption is super¬ 
posed. These observations are in accord with the conclusions arrived at in 1882 and 
1885 upon evidence derived from the examination of solution spectra. 
DESCRIPTION OF PLATES. 
PLATES 32 and 33. 
The original photographs were taken on plates measuring 5x4 inches, on which the continuous rays 
and the cadmium lines appear dark. Good examples are figs. 6 and 7, where the cadmium spectrum is 
shown at the head of each series. The reproductions are from negatives slightly enlarged to about one 
and a half times the size of the originals. 
Particulars of figures will be found facing the plates. 
The emission spectrum being dark the absorption bands appear white, or approaching to whiteness, 
according to whether they are strong or weak bands. On figs. 1 and 4 it will be seen that instead of the 
pure emission spectrum appearing at the head of the series a trace of benzene vapour gained access to the 
tube interposed between the spark and the slit of the spectrograph, when air was passed into it, and the 
absorption bands due to this trace of benzene are very distinctly shown. The reproductions do not 
compare in delicacy with silver prints taken from the original negatives, owing to a granulation resembling 
that of a lithograph, which somewhat impairs their definition. 
