OF THE VAPOURS OF BENZENE AND ITS HOMOEOGUES. 
485 
within the benzene and the toluene molecules, but as the bands of toluene are shifted 
into rays of longer wave-length, or less oscillating frequency, and are more widely 
separated, this effect must be attributed to the weighting of the molecule by the 
addition of CH 2 to the benzene. To compare the spectra properly, benzene at 50° C. 
and toluene at 80° C. should be chosen, since both substances at these temperatures are 
30° C. below their boiling-points. 
Grebe has compared the ethylbenzene spectrum with that of toluene and found the 
bands of the one to be contained in the other. 
The Homologues of Benzene and Toluene. 
The principal homologues of benzene and toluene* were studied and their spectra 
measured and compared. The hydrocarbons were obtained on different occasions from 
Kahlbaum and were found ±o have the correct boiling-points. They were redistilled 
and their boiling-points taken immediately before being used. The following is 
a list of these substances. Their boiling-points were constant:— 
Ethylbenzene . 
°C. 
boiling-point 136 5 
at 
mm. bar 
766. 
Dimethylbenzenes, or xylenes — 
1 : 2 or ortho . 
,, 144 
?? 
763. 
I : 3 ,, met a . 
138 
? ? 
765. 
1:4,, para . 
„ 138-5 
?? 
765. 
Methyl-propyl benzene 1 : 4, or cymene . 
i—' 
?? 
769. 
Trimethylbenzene 1 : 3 : 5, or mesitylene 
165 
?? 
758. 
The leading features of the three xylenes, cymene, and mesitylene, at different 
temperatures and constant pressure, are spectra with comparatively few bands or 
groups of bands; they have not been reproduced and therefore a detailed description 
of the photographs is given. They are each distinguished by a most powerful 
absorption of all rays at elevated temperatures, extending from about W 2800 to 
2340, but on lowering the temperature the rays between these limits are transmitted, 
at first only feebly, and more strongly subsequently. The highest temperatures at 
which the substances were examined are described first in each case. 
Description of the Spectra. 
Ortho-xylene vapour. (Table XII., p. 512.) 
At 121° C.—Complete absorption commences abruptly at approximately A. 2800, 
which is so intense that the strong cadmium lines are extinguished. This absorption 
* See also Pauer, ‘Wiedemann’s Ann.,’ 1897, 61, pp. 363-379; W. Friederichs, ‘Zeitschr. f. Wissen- 
schaftl. Photogr.,’ 1905, vol. 3, p. 154; L. Grebe, loc. cit., 1905, p. 363. 
