266 Hartley — The Action of Heat on the Absorption Spectra and 



permanganates, ^solid and in solution, cobalt glass, cobalt hydrate, cobalt chloride, 

 blue, solid, the same red, solid. Cobalt chloride in water, hot and cold, cobalt 

 chloride in alcohol, in strong and dilute solution. Uranium nitrate solid, and 

 dissolved in alcohol and in water. 



The change in the spectrum of cobalt chloride, he mentions as one of the most 

 remarkable examples which had come under his observation. He examined also 

 various chromium compounds. Vogel does not appear to have sought the cause of 

 the change in the spectra of these substances. Von Babo* tried the iniiuence of 

 dehydrating substances and of heat on cobalt chloride solutions. A few drops of 

 a concentrated solution of cobalt chloride, when added to a solution of calcium or 

 magnesium chloride boiling at 114% became blue at ordinary temperatures ; a more 

 dilute solution boiling at 108' gave a red liquid under the usual conditions, but 

 became blue on boiling. 



A solution of common salt mixed with the cobalt compound is red until heated. 

 With zinc chloride the same alteration did not take place, a fact explained by 

 von Babo by the probable formation of a double salt being supposed. If we 

 regard the matter from the point of view that dissociation takes place in these 

 solutions, then the addition of a dehydrating substance facilitates this change in the 

 solution, and the action of calcium and magnesium chlorides is easily understood. 

 In the case of zinc chloride, if we presume, as von Babo suggested, that a double 

 salt is formed, we can just as readily understand that cobalt-zinc chloride does not 

 undergo dissociation, or that the zinc chloride of the compound does not favour it, 

 but on the contrary prevents it, for its very action, as a dehydrating substance 

 attracts water to the molecule and retains it, so that the hydrated zinc-cobalt 

 chloride molecule is not dissociated, or, in other words, the metallic haloid 

 compound is not separated from its combined water at or about 100 C, when 

 in solution in water. 



* Jahrcsbericht, 1857, p. 72. 



