OF THE VAPOURS OF BENZENE AND ITS HOMOLOGUES. 
517 
Table XVIII.—Table of Wave-length Measurements of the Heads of Strong Bands 
which appear to be Common to Benzene aiid its Homologues. 
Abbreviations:—£, feeble; n., narrow 
Benzene. 
Toluene. 
Ethylbenzene. 
o-Xylene. 
m-Xylene. 
yi-Xylene. 
Cymene. 
Mesitylene. 
*2670 
2670 
2667 
2668 
2670 
2672 
2639 
2638 
2639 
2639 
2642 
2641 
2642 
— 
2634 f. 
2634 
2635 
2636 f. 
2634 
2630 
2634 
— 
*2608 
2607 
2607 
2609 
2609 n. 
2611 
— 
— 
*2603 
2605 
2605 
2605 
2606 n. 
2602 f. 
— 
— 
*2595 
2594 
2594 
2594 
2595 
2597 \ 
2593 
— 
*2590 
2589 
2586 
2586 
2590 f. 
2588 J 
2587 
— 
*2540 
2542 
2543 
2542 
2543 
— 
— 
— 
*2529 
2532 
2532 
— 
2532 
— 
— 
— 
*2471 
2472 
2473 
— 
2473 
— 
— 
— 
* The bands marked thus are those which appear strongly throughout this group of hydrocarbons. 
On the Relation of the Absorption Bands in the Spectrum of Benzene Vapour at 
Atmospheric Pressures and Temperatures-to the Bands Observed in Solutions 
of Benzene in Alcohol. 
Pauer compared the groups of bands in the absorption spectrum of benzene with 
the bands observed in the liquid itself and in solution in alcohol. The total number 
of bands observed in the vapour was 29, in a column 10 cm. long, containing 
atmospheric air charged with benzene vapour at a temperature of 20° C. These he 
divided into four groups, with three additional bands commencing as it were a fifth 
group. He assigns five bands to the liquid and to the solution respectively. He 
describes the spectrum of the vapour as being composed of fine lines. That they have 
this appearance is probably due partly to the low temperature and small quantity of 
the benzene examined, and partly to the small dispersion of his instrument, which 
was fitted with one Cornu’s quartz prism of 60° and quartz lenses of short focus, 
achromatised with fluor spar. The angle of inclination of the photographic plate to 
the normal of the lens would thus be not much less than 90°, whereas with 
uncorrected lenses it would be approximately 22°. 
The greater the length of focus of the lenses, if of plain quartz, the greater is the 
separation of the lines; thus, if the focus for the line I), X 5893, be about 900 mm., 
that for Cd 26, A. 2144, will be 776 mm., making a difference of about 5 inches, and on 
photographs taken with a dispersion of only one quartz prism and such lenses the 
linear distance measured between Cd 17, X 2748, and Cd 26, X 2144, is 87 nun., the 
last line showing a fine reversal. This affords evidence of accurate focussing and 
perfect definition. 
With a quartz train consisting of two half prisms of 30° of opposite rotation to a 
