Chemical Constitution of Saline Solutions. 299 



the green hydrated nickel bromide ; the green dihydrate, and brown hexa-hydrate 

 of cobalt iodide. 



The first is a familiar exami^le : it is crimson, and readily changes to purple 

 by loss of water on rise of temperature ; the second salt consists of a rich grass- 

 green aggregation of crystals. When dissolved in as little water as possible a 

 similar dark green solution is formed. The latter salt occurs in dusky reddish* 

 brown hexagonal prisms, a saturated solution of which at 16° and 20° has a similar 

 though deeper colour. Other illustrations of this are afforded by copper and 

 nickel compounds. There are, however, a few salts, and these are generally very 

 soluble substances, which do undergo sensible alterations in colour, which altera- 

 tions have been observed in each case to correspond witli that produced by a 

 state of partial or complete dehydration. Such salts are the cupric chloride an(? 

 and the hydrated nickel bromide. 



CuCl2'2HoOt is a pale blue salt; but its solution is grass-green, similar 

 in colour to that of the monoliydrated, CuCIa'HoO. When exposed to the action 

 of moist air, the blue crystals absorb water, gain weiglit, and, in doing so, turn 

 green, and become moistened with a solution of the salt. A green solution ma}^ 

 be obtained which will not deposit the salt CuCla'SHoO, unless evaporated to 

 such a degree that the whole liquid becomes nearl}' solid. This salt can scarcely 

 be contained in the solution ; it must be formed from the liquid as the salt 

 solidifies. 



CuBr2"5H20 is of a golden green colour ; its solution in water is intensely dark 

 brown, of the same colour as a solution of the compound CuBr2 when dissolved in 

 alcohol or in a minimum of water. The brown solution can be obtained in such a 

 condition that it does not deposit the salt CuBro'5H20, but only the anhydrous 

 salt CuBr2. Tlie salt NiBro'CHjO undergoes a similar change. 



It may here be remarked that whether the brown solution of cupric bromide 

 deposits anhydrous salt or the hydrate CuBr2*5H20, depends upon the tem- 

 perature. 



The following experiment shows how the hexahydrated cobalt chloride may 

 be partially dehydrated by solution in a solvent other than water : — 



The crystals were dissolved in strong alcohol of 98 per cent., or there- 

 abouts, forming a beautiful deep blue solution ; on partial evaporation, without 

 heat, crystals were deposited, which, on analysis, proved to be the original 

 hexahydrate. This is proved by the following analytical numbers : — At a 

 temperature of 100°, 2 grs. of the salt lost 0*605 grs. of water, corresponding to 

 30-25 per cent. The conversion of the hexahydrate into the dihydrate at 100° 



* A statement in the last edition of Watts' Dictionary is, in this particular, inaccurate. The salt is 

 described as being red. 



t Jour. Chem. Soc, vol. 13, p. 256, 1875. 



