524 
DR. WALTER NOEL HARTLEY ON THE ABSORPTION SPECTRA 
Comparison of the Wave-lengths of the Heads of Bands in Solution and Vapour 
Spectra yielded by the same Quantity of Benzene (see fig. 7, Plate 32). 
Differences in 
wave-lengths 
between the 
Heads of bands. 
Differences. 
heads of successive bands. 
Number of 
o 
Ansrstrom Units. 
Angstrom 
Units. 
band. 
Solution. 
X 
Vapour. 
X. 
Solution. 
Vapour. 
2682 
2670 
12 
I. 
40 
40 
(1) 
j 2657 
\2642 
2630 
12 
11 . 
25-5 
(2) 
| 2630 
\2616-5 
III. 
16-5 
40 
2614 
\ 2600 
2590 
10 
IV. 
61 
67 
2539 
2523 
16 
V. 
59 
57 
2480 
2466 
14 
VI. 
50-5 
51 
2426•5 
2411 
15-5 
VII. 
2376 
2360 
16 
VIII. 
34 
2326 
IX. 
47 
2279 
X. 
Note.—(1) A feeble band, the width of which is measured; it appears to correspond with a feeble band 
in the vapour, X 2630. (2) These two bands appear to be the result of the splitting of a broad band, the 
head of which is at X 2600. It corresponds to the vapour band at X 2590. 
It will be remarked that in the first series of bands taken from solution spectra 
there are only seven, in the second there are eight, and in the vapour spectrum there 
are nine bands. Here again it will be seen, also, that the differences between 
corresponding bands in solution and vapour spectra respectively, as was shown on 
p. 519, amount to from 10 to 1(3 Angstrom Units, which represent the shift towards 
the visible spectrum of different bands measured in solutions. . 
Summary and Conclusions. 
The variations in the vapour spectra at different temperatures and pressures may 
be explained by the fact that there are two kinds of absorption which are sharply 
defined and may be differentiated ; these are distinguished by the terms general and 
selective absorption. First, by rise of temperature, the general absorption is 
