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



causes a loss of 30-29 per cent. On further heating the salt to 140°, a further 

 loss of 0-185 grs. occurred, and the colour of the salt changed to a lavender blue 

 in consequence of complete dehydration. 



I have observed that the solid anhydrous salt becomes blue when hot, but 

 becomes purple when cold. 



It may here be noted that a specimen of cobalt chloride, CoCL'GHaO, well 

 crystallized, has yielded the following numbers : — 



When heated to 100° the loss of water was, in two analyses, (1) 39-06 per 

 cent.; (2) 38'92 per cent: consequently, as the hexahydrate contains 45-5 per 

 cent., the compound dried at 100° contained 6-5 per cent, of H2O. This corre- 

 sponds to a salt, with the formula C02CI4-H2O, or (CoCl2)2'H20, a hydrate which 

 has not been previously obtained. 



The blue solution in alcohol was proved not to be an alcoholate, but either the 

 dihydrate or the anhydrous salt, and most probably the former. This is the 

 nature of the evidence on that point. Equal quantities of 2 grs. of CoCl2*6H20 

 were dissolved in absolute alcohol, and made up a volume of 5c.c., but previous to 

 solution one of these was completely dehydrated at 150°. In this second case, 

 scarcely the whole of the salt dissolved, on account of its smaller solubility, 

 though the other solution of the hydrated salt was easily made. The an- 

 hydrous salt, CoCL, dissolved with a superb ultramarine tint, which, on getting 

 more concentrated, became darker and indistinguishable from the solution of 

 the hydrated salt. The spectra of the two solutions were not identical, 

 though the difference between them was but slight ; and everything pointed 

 to the conclusion that the hexahydrate, by solution in alcohol, is converted 

 into the dihydrate. 



The black cobalt iodide, CoL, dissolves in alcohol with an indigo blue colour ; 

 the vivid green bromide, CoBro, also with a blue colour ; and the yellow cupric 

 chloride, CuCl2, with a brown colour. Exposure of these alcoholic solutions to 

 the air yields hydrated compounds ; while evaporation in vacuo, or over oil of 

 vitriol, leaves the anhydrous salts. 



The brown hexahydrated cobalt iodide, Col2-6H20, is converted into the 

 green, Col2-2H02, when the former is dissolved in alcohol. The brown salt, 

 Col2'6H20, is a comparatively stable hydrate ; it forms a brown solution in 

 water, which becomes green at 50°, of the same colour as that of the solid 

 C0I2-2H2O, and also of the alcoholic solution. 



The only purely physical change in the spectra effected by rise of temperature 

 is a darkening of the solution ; but, in these instances, the salts in solution must 

 necessarily undergo a change in chemical constitution, which is recognised as 

 dehydration. 



Since the date of the foregoing experiments, the changes in colour of hydrated 



