482 
DR. WALTER NOEL HARTLEY ON THE ABSORPTION SPECTRA 
decreases. The groups of vapour bands appear to be caused by the overlapping of 
two or more similarly constituted spectra differing in intensity. The bands described 
as strong which fall within this description number fifty-four, there being twenty- 
seven in each of the two spectra. In addition to these there are thirty feeble bands, 
which also fall into two series of similar groupings, but with less regularity. To 
render this evident they have all been mapped according to a scale of inverse wave¬ 
lengths, or oscillation frequencies, with the result that the entire number of bands 
observed between the limits of 12°*7 C. and 25° Cl, under a pressure of 759‘5 mm., is 
resolved into four spectra (see map, p. 484), of which two are strong and two are weak. 
The Spectrum of Toluene. 
The specimen of toluene was obtained from Kahlbaum. It boiled with no 
variation at Ill 0 Cl, with the barometer at 763 mm. Rise of temperature from 
10° C. to 90° C. has very little effect upon the toluene spectrum (see fig. 5), that is to 
say, the constitution of the groups of bands in rays less refrangible than X 2400 is 
^scarcely altered, and the number of bands is not increased, they only widen, but the 
general absorption of the more refrangible rays increases, and the change is decidedly 
greater at 100° Cl Variations in pressure at 11° C. likewise scarcely affect the bands 
(see fig. 6), but the alteration is chiefly in the rays more refrangible than X 2300. 
It is very remarkable that the series of bands and their particular grouping, which is 
characteristic of the benzene molecule, is greatly modified by the very simple 
substitution of ( 'I I 3 for H. It was remarked by Grebe that the spectrum is quite 
different from that of benzene, but on careful observation of the different temperatures 
and pressures at which benzene and toluene have been photographed this is seen to 
be not quite correct. The different temperatures at which the spectra were photo¬ 
graphed were 10°, 30°, 40°, 50°, 60°, 70°, 80°, 90°, and 100° C. (fig. 5), with a 
barometric pressure of 751 mm. This series shows to perfection that the general 
absorption gradually increases with the temperature. There is a very feeble absorp¬ 
tion of the continuous rays by toluene at all pressures, the last line of cadmium, 
X 2144, being quite cut off, and all lines more refrangible than wave-length 2471 are 
weakened. 
The following grouping of the toluene bands shows every band measured from that 
substance. The numbers in brackets represent six bands observed by Grebe which 
do not appear on my photographs :— 
