1912-13.] The Absorption of Light by Inorganic Salts, No. IX. 137 
XII. — The Absorption of Light by Inorganic Salts. No. IX. : Solu- 
tions of Copper, Nickel, and Cobalt Salts in Alcohol and in 
Acetone. By R. A. Houstoun, M.A., Ph.D., D.Sc., and A. H. 
Gray, M.A., B.Sc., Research Student in the University of Glasgow. 
(MS. received February 10, 1913. Read March 3, 1913.) 
The absorption spectra of the copper, nickel, and cobalt salts investigated 
in this series do not seem to depend so much upon the acid radical as on 
the water of solution. The latter evidently plays a very important role, 
and possibly complicates matters. It was therefore resolved to determine 
the molecular extinction coefficients in alcohol and in acetone to see 
whether conditions were any simpler there. It may be stated in advance 
that they are not. 
As these solvents mix with water and with each other, their use seemed 
to promise an application of the law of mass action similar to that dis- 
cussed in the sixth article of this series. This was an additional reason 
for studying them. 
Some determinations of molecular extinction coefficients in the ultra- 
violet for solutions in alcohol have already been made in the eighth paper 
of the series; the work described in this article is wholly in the visible 
spectrum. 
As the spectrophotometer used in previous work was in constant use, 
it was necessary to get a new instrument, and a convenient and inexpensive 
one was made to the design of one of us by Messrs R. and J. Beck, at the 
expense of the Carnegie Trust for the Universities of Scotland. It was 
exhibited at the recent Optical Convention in London, and is shown in the 
photograph on p. 138. 
The light from the right falls first upon a prism of glass and Iceland 
spar of the design described in a previous paper,* and is divided into two 
beams polarised in planes at right angles to one another. These beams 
are focussed by a lens on the slit, and pass through a polarising prism 
mounted with a divided circle, before reaching the latter. The sharp edge 
of the glass and spar prism is vertical and the slit horizontal. The spectro- 
scope was a Beck-Rafferty grating one. The wave-length could be read 
* Phil. Mag. (6), 15 (1908), p. 282. 
