256 Hartley — TJie 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, 1 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 -^\-^\h of the spectrum from \ 768 below A, to X 410 or h. 



The wedge-cells were cut out of thick pieces of glass in the following manner. 

 A block of glass \ in. by \ in. by 3 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 f in., the thickest part of the hollow being also \ in. 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 xV^hs in. by f in. by 3 in., the thickest part being able to contain a layer of 

 liquid T^tlis 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 suu was not available consisted 

 of an argand burner of the ordinary construction for gas, into the hollow centre of 

 which a narrow tube of -f-Vth 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. 



