50 
Proceedings of the Royal Society of Edinburgh. [Sess. 
Till. — The Absorption of Light by Inorganic Salts. No. VI. : The 
Cobalt Chloride Colour Change. By Alex. R. Brown, M.A., 
Carnegie Scholar in the University of Glasgow. Communicated by 
Dr R. A. Houstoun. 
(MS. received December 1, 1911. Read January 22, 1912.) 
In No. II. * of this series of papers, measurements were given of the 
striking change shown by cobalt chloride when an aqueous solution was 
heated or was altered in concentration, and it was announced that a more 
accurate investigation was being made in the hope of finally determining 
its cause. Chemists suppose the cause to be the dehydration of the hexa- 
hydrate, but the matter is not beyond doubt. 
First of all, aqueous solutions were used. Except in regard to the 
heating arrangements the methods and apparatus were those used in the 
previous research. The heater in this case was a rectangular copper casting 
8 cm. long, 2‘5 cm. broad, 4'5 cm. high, which at one end was attached to 
a tin in which water was boiled. At a distance 1*2 cm. from the other 
end a rectangular hole was cut down through the casting, of dimensions 
such that it was just filled by the cell containing the solution to be heated. 
At each side of the hole from the base upwards a part of the casting 
1*6 cm. high and IT cm. broad was cut away. This permitted the 
transmission of light horizontally through the lower part of the vertical 
cell, which was of the ordinary rectangular type, and obviated the use of 
mirrors. As in the previous research glasses were inserted in the cell to 
reduce the thickness of the layer of solution examined, and the temperature 
of the solution was read by a thermometer. 
By this method a steady temperature of about 84° could be obtained. 
But readings could easily be taken at points between room temperature 
and 84°, it being possible by judicious application and withdrawal of the 
bunsen burner to keep the temperature of the solution within a degree or 
two of any selected point. The time taken by the solution after the 
application of the bunsen to reach the highest temperature was slightly 
over 20 minutes, and on withdrawal of bunsen the times taken to cool 
to 50° and to room temperature were over 40 minutes and about 180 
minutes respectively. Ample opportunity was thus afforded for taking 
readings for intermediate temperatures. 
* Proc. Roy. Soc. Edin ., xxxi. p. 530 (1911), 
