476 
DR. WALTER NOEL HARTLEY ON THE ABSORPTION SPECTRA 
when a powerful spark is passed between poles of aluminium immersed in water. 
The exposure necessary in Friederichs’ experiments was from 12 to 21 hours, in 
Grebe’s from f to 1 hour. Only the maximum of absorption of the bands was 
measured by these investigators and not their width, but the widening of the bands 
is an important feature in the spectrum when changes of temperature and pressure 
occur. I have re-investigated the vapours of the aromatic hydrocarbons recently by 
means of the continuous rays emitted by condensed sparks passed between various 
electrodes, but for measurements in the ultra-violet the cadmium continuous spectrum 
which accompanies the lines of that element has alone been employed, because it has 
been found, on the whole, to yield the most satisfactory results. # 
The vapours of benzene and some of its derivatives have been examined (l) at 
different temperatures and constant pressure, and (2) at different pressures, the 
temperature being constant. The several different spectra photographed prove the 
correctness of the previous measurements of Pauer, Friederichs, and Grebe, though 
these do not show complete agreement among themselves. 
Thus the band at wave-length 2694 (Friederichs) is a real one and not, as Grebe 
believed, a mere marking or scratch on the photographic film ; also the band at 2631 
observed by Friederichs is undoubtedly a benzene band; so likewise are those at 
2633 and 2638 observed by Grebe, but believed by him to be due to an impurity. 
Experimental. 
The benzene was a portion of 2 litres obtained from Kahlbaum which had been 
crystallised and was free from thiophene. A quantity of 250 c.c. was recrystallised, 
the crystals being pressed between folds of blotting pajier, melted and redistilled. 
Its boiling-point was constant at 81° C. and 763 mm. barometric pressure. 
The Absorption Spectra at Varying Temperatures. 
An arrangement was used similar to that employed by Pauer, with the exceptions 
that the tube was of bronze 150 mm. long, fitted into a rectangular water-bath which 
was deep enough to hold the small flask of benzene as well as the immersed tube; 
also the metal tube was closed at each end by a quartz plate cemented on to a flange 
with seccotine, or fish glue, and could be exhausted, partially or completely, at any 
temperature. 
The bath in different experiments, according to the temperature required, was filled 
with water or glycerine. Brazed to the bronze tube were two smaller tubes for inlet 
and outlet of air or a neutral gas. The inlet was connected with the side tube of the 
flask containing the benzene, the outlet with a reversible aspirator which holds 
* ‘Roy. Soc. Proc.,’ 190G, A, vol. 78, pp. 243-245. Pauer arrived at the same conclusion, see footnote 
on p. 365 loc . cit . 
