518 
DR. WALTER NOEL HARTLEY ON THE ABSORPTION SPECTRA 
single prism of 60°, the lenses being of 325 mm. focus, the linear distance between 
the same lines measured on a photograph is about 55 mm. (See ‘ Chem. Soc. Trans.,’ 
1898, vol. 73, p. 695.) 
The absorption spectra of a series of solutions of benzene were photographed by 
Hartley and Dobbie with the quartz train of three prisms, and it was observed that 
the bands were sharpest where the rays are of shorter wave-length, and are gradually 
weakened in the opposite direction as if they were composed of groups of lines or very 
narrow bands, closer together and stronger on the side of the least refrangible rays. 
Both Friederichs and Grebe have compared the positions of the heads of the bands 
in these spectra with respect to the heads of the groups in the vapour spectra of the 
same substance as measured by them. Shortly described, the differences between 
the spectra of the solution and the vapour are such that the former are very broad 
and few in number, the latter multitudinous, very narrow, and arranged in groups. 
The corresponding bands and groups in the spectra of the two different states (lifter 
in the bands of the solution being shifted by a definite number of wave-length 
differences towards the less refrangible rays, both the bands and groups of bands 
be in o’ degraded in the same direction. This is shown on reference to fig. 7, where 
the spectra of exactly the same quantity of benzene, in the state of vapour and of 
solution, are photographed on the same plate, at the same time, and with the same 
exposure. Here it may he remarked that the displacement is not at all evident from 
Pauer’s measurements, which are here quoted. 
Heads of bands . . 
I. 
II. 
III. 
IV. 
V. 
A. 
A. 
A. 
A. • 
A. 
Solution. 
Vapour. ..... 
2590 
2592 
2535 
2528 
2480 
2467 
2420 
2412 
2360 
2362 
Of the more recent series of numbers taken for comparison, the measurements made 
by Grebe exhibit a greater regularity in their differences than those quoted by 
Friederichs. It will be convenient to name the spectra of the vapour vapour spectra , 
and those of solutions solution spectra. As neither the quantity of substance in the 
state of vapour, nor the barometric pressure at the time, were taken into account by 
Palter, Friederichs, and Grebe, and as the two latter experimenters do not mention 
the temperature at which the benzene was vaporised, it is difficult to make a strict 
comparison with their measurements ; but it will be seen that the regularity in the 
differences is more striking if the benzene vapour spectrum at 12°'7 G. and barometer 
759'5 mm. is compared with the alcoholic solution containing very nearly the same 
quantity of benzene, since the conditions under which the former were photographed 
