OF THE VAPOURS OF BENZENE AND ITS HOMOLOGUES. 
487 
another rapidly as the temperature rises, and are rendered indistinct by an increasing 
general absorption. The correspondence of the one spectrum with the other is best 
seen by superposing one on the other ; the groups are then easily identified. 
Para-xylene has a spectrum which differs to some extent in constitution from that 
of meta-x ylene. The groups of bands are situated in the same region as those in the 
meta-xylene ; but bands appear in the para- at low temperatures which are seen only 
at higher temperatures in the meta-derivative. 
The para-xylene shows a number of bands in the less refrangible region at higher 
temperatures which do not appear at all in the meta-xylene spectrum. In character 
the bands are more diffuse than in the meta-xylene, and not so sharply defined. 
Ortho-xylene has an absorption spectrum which contains groups of bands similar to 
those in the meta- and para-isomers, but they are much more diffuse, and at first 
sight the spectrum bears but a slight resemblance to that of benzene. 
Thus the similarity between the spectra of ortho-, meta-, and para-xylene and that 
of benzene is greatest with meta-xylene; it decreases with para-xylene, and is 
decidedly the least in the case of ortho-xylene. From the chemical point of view 
of position isomerism it is not easy to account for this difference, since it would rather 
be expected that either the ortho- or the para-xylene would most nearly resemble the 
ethylbenzene or toluene. 
Ethylbenzene has a spectrum closely resembling that of benzene, and it occupies a 
position between toluene and meta-xylene, which is its natural position. 
The spectra of ethylbenzene and toluene are almost identical, both as regards the 
number of bands and their wave-length ; where a difference in wave-length occurs, 
the band in the ethylbenzene spectrum is generally somewhat shifted more towards 
the red than the corresponding toluene band, which again is to be attributed to the 
weighting of the molecule. Tables X. and XI., pp. 510, 511. 
Ethylbenzene shows a much greater similarity to toluene than toluene does to 
benzene, the substitution of CH 3 for H in the side chain producing but a slight 
alteration in the spectrum. 
A Comparison of the Absorption Spectra of the same Quantity of Benzene Vapour 
under Different Conditions as to Temperature and Pressure. 
Photographic plate I., spectrum 1, fig. 1. (See Plate 32.) 
The spectrum is produced by 0 - 0179 grm. of benzene at 12° - 7 C. and 53 mm. of pressure. 
Photographic plate III., spectrum 2. Not reproduced. 
This is produced by 0'0179 grm. of benzene at 100° C. and 69 mm. pressure. 
The two spectra differ widely; at 12 0, 7 C. the spectrum is divided into groups of 
well-defined absorption bands which extend through the region lying between wave- 
