538 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XXXVIII. — The Absorption of Light by Inorganic Salts. No. III. : 
Aqueous Solutions of Nickel Salts in the Visible Spectrum 
and the Infra-Red. By R. A. Houstoun, M.A., Ph.D., D.Sc., 
Lecturer on Physical Optics in the University of Glasgow. Com- 
municated by Prof. A. Gray, F.R.S. 
(MS. received March 8, 1911. Read June 5, 1911.) 
The methods and the apparatus were the same as in the two previous 
articles of the series and hence need not be described, the only change 
being in connection with the galvanometer. Previously it had rested on 
the table, but when half the results in the infra-red recorded in this paper 
were obtained, the arrangement was changed ; it was suspended by three 
iron wires each about one and a half metres long from a bracket in the 
wall and hung with its three levelling screws clearing the table by about 
one centimetre. Between the table and levelling screws were placed loose 
wads of cotton wool for the purpose of damping any vibrations that might 
arise. With this arrangement the zero is very much less sensitive to 
vibration. With the lamp and scale at one and a half metres — the distance 
at which they are ordinarily used now — vibrations of the laboratory 
rarely cause the zero to move \ mm. either way. 
Some trouble was caused by the iron wires of the thermopiles rusting 
through. 
The salts were obtained from Kahlbaum. The nitrate, sulphate, and 
chloride, which were supplied as Ni(N0 3 )6H 2 0, NiS0 4 7H 2 0, and NiCl 2 6H 2 0, 
gave no trouble. The bromide was a yellowish-brown powder with a very 
slight greenish tint in some parts. By heating so as to drive off the water 
of crystallisation it was found to be NiBr 2 3H 2 0. It dissolved very readily 
with froth, gave a turbid green solution, and on filtering left some yellow 
precipitate on the paper in analogy with cobalt bromide. Dilute solutions 
also became slightly turbid on standing some time. The iodide was of 
an iron-black colour. By driving off the water of crystallisation it was 
found to be NiI 2 6H 2 0. It dissolved frothily, and the solution looked 
turbid. On filtering, a brown solution was obtained and a chocolate-coloured 
precipitate similar to the precipitate in the case of cobalt iodide. The 
weight of the precipitate could be neglected in comparison with the weight 
of the dissolved salt. The solution did not keep well. For example, in 
three months in a normal solution exactly half the dissolved salt decom- 
