OF THE VAPOUES OF BENZENE AND ITS HOMOLOGUES. 
521 
one broad band and two or more narrow ones, or a single broad band, takes the place 
of the several bands characteristic of this hydrocarbon. Isomeric hydrocarbons differ 
from one another, each having its own peculiar spectrum, even when the isomerism is 
due to the positions of the substituted hydrogens in the benzene ring. That which 
has been observed with respect to solution spectra has been ascertained by the 
foregoing experiments to apply to vapour spectra. This was recognised also by 
Pauer. It is more particularly evident in the vapour spectra of the three xylenes, 
in cymene, and in mesitylene, that several groups of narrow bands are merged into 
one broad band, the measurements of which, as regards breadth, intensity, and position 
in the spectrum, are different for each substance; at the same time the general 
absorption is shifted towards the less refrangible rays and is sometimes greatly 
increased. This suggests not only a lowering of the kinetic energy of the molecules, 
but some irregularity in their motion which gives rise to an increased number of 
shocks or encounters (see Tables XII., XIII., XIV., and XVI.). 
After a careful examination of the tables of wave-length of vapour bands and 
comparing them with the photographs, it does not appear to be evident that the 
greater molecular weight of a benzene homologue shifts all the bands towards the red, 
though it is apparent that certain of the bands are so shifted ; it is also clear that 
certain groups of bands are common to benzene, toluene, ethylbenzene and o-xylene, 
and that these are the bands affected. But the weighting of the molecule, besides 
diminishing the number of the vapour bands, causes the general absorption to extend 
further towards the rays of longer wave-length. At constant pressure the effect of 
rise of temperature might be expected to cause an increased number of molecules to 
cross the path of the rays, which is the same in effect as increasing the quantity 
of substance, or lengthening the column of vapour; reduction of pressure would 
therefore be expected to have an opposite effect, by reducing their number and 
increasing the mean path of the molecules—the absorption bands, if caused by 
vibrations within the molecule, would as a consequence become more distinct. This 
is, in fact, what is actually seen in the least refrangible groups, between XX 2750 and 
2500, particularly when benzene vapour is maintained at 100° C. and the pressure is 
gradually reduced to 4 mm. (See fig. 2 ; for the wave-lengths also Table VI.) The 
quantity of benzene in the 79 c.c. is only O'OOll grm. in weight, at 4 mm. pressure. 
It has been experimentally shown that increase of mass, of density, and of tempera¬ 
ture operate in the same manner upon the rays entering a column of benzene vapour : 
in the first place, the general absorption is increased by an extension towards the 
visible rays and it is also intensified ; in the second place, the bands of absorption 
broaden, become diffuse, and coalesce. As the bands attributed to the internal 
vibrations of the parts of the molecule in different isomeric hydrocarbons become 
diffuse and coalesce, partaking of the character of the general absorption, and as this 
is not confined to one part of the spectrum, namely to the less refrangible or trans¬ 
mitted rays, but is a selective absorption, it cannot be due to simply a loading of the 
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VOL. C'CVIII.-A. 
