256 Harttey—The Action of Heat on the Absorption Spectra and 
He observed that the haloid salts of cobalt differed in respect to the position of 
the absorption bands inasmuch as the bromide exhibited a band nearer to the red 
than the chloride, while that of the iodide was moved still further down. The 
effect of heat and that of substances capable of combining with water on aqueous 
solutions of cobalt chloride are identical, as both tend to form the banded spectrum 
of the anhydrous chloride when viewed in solution. 
Method of Experimenting. 
In 1873-74, I examined a number of salts both in the solid state and in solution 
at different temperatures, but generally at 20° and 100°C. Wedge-shaped cells of 
glass were used to hold the liquids, and these were heated in an air-bath provided 
with two sides of glass, so that light might be passed through the cells and pro- 
jected on to the slit of a spectroscope provided with a single flint-glass prism. The 
measuring arrangement was a divided circle over which the telescope moved. In 
measuring dark bands, which gradually merged into bright spaces, the cross-wires 
in the eye-piece were placed in this position X, and the telescope was moved so that 
they were only just obscured by the dark shading to the extent of one-half the field 
of view. The accuracy attainable with consecutive readings was in many cases 
such as to limit the error to about one minute of arc between 35° and 39° 39’, 
or about =4,th of the spectrum from \ 768 below A, to \ 410 or h. 
The wedge-cells were cut out of thick pieces of glass in the following manner. 
A block of glass 3 in. by 2 in. by 8 in. had a slice off one of its solid angles ground 
down to 45° for the space of an inch in the centre of the block, and the surface of 
this was polished. By placing a piece of plate-glass against one of its sides, and 
fixing it by a metal clamp, a wedge-shaped vessel or hollow prism of 45° was 
formed, having a depth of 3 in., the thickest part of the hollow being also3in. The 
refraction of the liquid prism was thus compensated by the solid prism of glass. 
Some thinner wedges also were made for darkly coloured liquids. ‘Their dimensions 
were =3;ths in. by 3 in. by 3 in., the thickest part being able to contain a layer of 
liquid ;2;ths of an inch in thickness. 
The air-bath was provided with a thermometer and a Geissler gas-regulator 
or thermostat, which were placed close to the cell containing the solution under 
examination. An efficient source of light when the sun was not available consisted 
of an argand burner of the ordinary construction for gas, into the hollow centre of 
which a narrow tube of ~;th of an inch in diameter was introduced. The end of 
this tube was closed, but the periphery was pierced with small holes. This tube 
conveyed oxygen into the flame, and yielded a very brilliant light when needed. 
Most of the salts examined were prepared by myself; some were quite new, and a 
complete examination of their properties was necessary. 
