OF THE VAPOURS OF BENZENE AND ITS HOMOLOGUES. 
525 
broadened and extended towards the less refrangible rays, the effect of which is to 
cause not only the narrow absorption bands but also the emission spectrum of 
metallic lines and diffused rays to become obscured, while the feeble narrow bands 
become strong and increase in number in that part of the spectrum which is 
unaffected by the general absorption. By reduction of pressure, temperature being 
constant, the general absorption is decreased, so that, consequent upon a diminution 
in the quantity of substance under observation in the tube, the narrow absorption 
bands become more numerous, but neither by rise of temperature nor reduction of 
pressure is the position of these bands altered. Secondly, the selective absorption is 
manifest in all the narrow individual bands and groups of bands, such as those that 
have just been referred to as unaltered in position by variations in temperature and 
pressure. The former, or general, absorption appears to be due to the translatory 
kinetic energy of the molecules. The selective absorption is to be explained by the 
vibrations of the atoms or atom-complexes within the molecules, so that they are not 
affected in the same manner as the translatory molecular movements by rise of 
temperature; they are not subject to displacement. When a larger number of 
molecules passes the slit, a greater number of the intra-molecular vibrations is brought 
into the field, and therefore feeble bands become intensified. Also bands not 
discernible at ordinary temperatures become apparent, and more intense as the 
temperature of the vapour rises. Moreover, by reduction of pressure and consequent 
greater duration of undisturbed motion of the molecules, the bands attributed to the 
intra-molecular vibrations become sharp and distinct. 
The selective absorption is best studied by raising the general absorption to a 
maximum (at 100° C.) and studying the spectra as modified subsequently by 
reduction of pressure. By proceeding in this manner the changes in the spectra due 
to the general absorption may be eliminated (see figs. 3 and 4, Plate 33). 
The Homologues of Benzene and Isomeric Substances. 
In toluene and ethylbenzene spectra it is shown (l) that there are bands and 
groups of bands apparently having the same origin as those in benzene, and they are 
similarly constituted, but by the weighting of the molecule they are shifted into rays 
of less refrangibility, that is to say, with reduced oscillation frequencies ; (2) there are 
bands which do not suffer displacement, but appear to be common to benzene and its 
homologues ; (3) by increase of temperature and reduction of pressure the homologues 
of benzene suffer less change in the constitution of their spectra than benzene itself. 
Thus the bands and groups of bands are not affected to the same extent, neither is 
their number so largely increased or their intensity so augmented. 
The principal changes in the spectra of ortho- and -xylene, occasioned by rise 
of temperature, are an increase in the general absorption over a wide range of the 
spectrum, which decreases the number of bands in parts of the spectra which are 
