530 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XXXVII. — The Absorption of Light by Inorganic Salts. No. II. : 
Aqueous Solutions of Cobalt Salts in the Visible Spectrum. 
By R. A. Houstoun, M.A., Ph.D., D.Sc., Lecturer on Physical Optics, 
and Alex. R. Brown, M.A., Thomson Experimental Scholar in the 
University of Glasgow. Communicated by Prof. A. Gray, F.R.S. 
(MS. received March 8, 1911. Read June 5. 1911.) 
As in the case of the work in the infra-red, the salts experimented on 
were the nitrate, sulphate, chloride, bromide, fluoride, and iodide, and they 
were obtained from Kahlbaum. Very little trouble was experienced with 
the sulphate, nitrate, and chloride. The salts were supplied as CoSo 4 7H 2 0, 
Co(N0 3 ) 2 6H 2 0, CoC1 2 6H 2 0. The fluoride was a pinkish powder sparingly 
soluble in water. It dissolved with formation of insoluble oxyfluoride 
which seemed to increase on standing. By heating the salt and driving 
off the water of crystallisation, it was found to be CoF 2 2H 2 0. The iodide 
was an extremely deliquescent green powder. A slight insoluble chocolate- 
coloured residue remained in the solution and was filtered off. By heating 
the salt it was found to have the composition CoI 2 2H 9 0. The bromide 
consisted of very deliquescent crimson lumps, and was found to have the 
composition CoBr 2 6H 2 0. 
The apparatus used was a spectrophotometer belonging to the Carnegie 
Trust for the Universities of Scotland, of a new type, which has already 
been fully described,* and which has been used for several photometric 
researches. In this instrument there are two slits illuminated by the same 
piece of the same incandescent mantle. Corresponding to the slits, there 
are formed two spectra, the one directly above the other, the intensities 
of which are matched by the rotation of a nicol in the eyepiece. 
The absorption was specified as before by the molecular extinction 
coefficient which is defined by the equation 
i = i 0 io- A cc *, 
where A is the molecular extinction coefficient, c the concentration of the 
solution in gramme-molecules per litre, d the thickness of the layer of 
solution through which the light passes, I 0 and I the intensities of the 
light before and after passing through the layer. 
The dilute solutions were examined first. The usual method was to 
take two glass cells the internal thickness of which was 1 cm. ; one was 
filled with water and the other with the solution. They were then placed 
one before each slit of the instrument, and the intensities of the light, 
which they transmitted, matched in two adjacent quadrants, the difference 
* “ A New Spectrophotometer of the Hiifner Type,” Phil. Mag. (6) xv., p. 282, 1908. 
