1910-11.] Absorption of Light by Inorganic Salts. No. IV. 557 
recent developments of the theory of electrons, and in addition it has, of 
course, not been borne out by experimental work. Some irregularities in 
the curves may be explained by the fact that we have not used the same 
concentration throughout the spectrum, though, as a rule, A does not vary 
much with c. But, owing to the wide limits within which A varies, it would 
be impossible to use the same value of c throughout the spectrum except 
at a very great cost of time and money. Owing to the want of suitable 
quartz cells we could not investigate the variation of A with c in the 
ultra-violet. It was noticed, however, that some of the bands in this 
region narrowed slightly with dilution. 
It was, of course, impossible for us to go further into the infra-red 
owing to the absorption of water, and into the ultra-violet owing to the 
absorption of quartz. 
The only quantitative work hitherto done on the absorption of light 
by aqueous solutions of cobalt and nickel salts has been noticed in one of 
the previous papers. It is that of E. Muller on nickel sulphate and nitrate 
in the visible spectrum. Much qualitative work has been done, some of it 
of great value. We reserve comparison of our results with those of other 
workers until we have data for more chemical elements. Meanwhile, for 
the complete bibliography of the subject we refer to Kayser’s Spectro- 
scopie, iii. pp. 409 and 416, and to The Absorption Spectra of Solu- 
tions , by Harry C. Jones and John A. Anderson, Carnegie Institution, 
Washington, Publication No. 110, 1909. We are continuing our work on 
other elements. 
In conclusion, a few values of pe/m, calculated by the formula * 
pe = -3170 A 0 (A 1 - X 0 ) 
m X 0 2 
may be of interest, p is the number of electrons or ions per molecule of 
salt, causing the absorption band under consideration, e being the charge 
on. and m the mass of, the electron or ion in question. A 0 is the value of 
A at the maximum, \ 0 the position of the maximum, and \ the wave-length 
for which A has half its maximum value. The formula, of course, holds 
only on the special assumption that the electrons or ions in question are 
not much influenced by the rest of the vibrating system, but in other 
cases the order of magnitude of pejm probably gives us a clue as to whether 
electrons or ions are in question. In any case, we cannot theorise more about 
the structure of the atom until we have more data. 
* R. A. Houstoun, “ On the Mechanism of the Absorption Spectra of Solutions,” Proc . 
Roy. Soc., 82 A, p. 606 (1909). 
