Chemical Constitution of Saline Solutions. 303 



When a salt is put into water, the water may (I) combine with it, (2) decom- 

 pose it, or (3) simply liquefy it. With one and the same salt it may depend upon 

 the temperature whether combination, decomposition, or simple liquefaction will 

 take place. The extent of chemical change depends upon — (a) tlie temperature, 

 and {b) the quantity of water present. The behaviour of salts with water is 

 different with different salts. But as combination is accompanied by heat evolution 

 and simple liquefaction by heat absorption, there is a particular temperature, as 

 Berthelot has shown,* for each salt, at which when dissolution takes place, heat is 

 neither evolved nor absorbed. Any temperature above this neutral point leads to 

 dissociation of the hydrate in solution which is manifested in coloured salts by a 

 change of colour. In point of fact, saline solutions of crystalline hydrates at 

 ordinary temperatures contain the molecules of hydrated salts, more or less liable 

 to dissociation, or even to complete dehydration. There may be several hydrates 

 present at the same time in the same solution, and even the anhydi'ous salt may be 

 present with them. Cupric bromide is a salt presenting a notable example, inas- 

 much as the black anhydrous compound can exist along with green pentahydrate, 

 CuBra'SHoO ; and cobalt iodide is another for the hexahydrate CoL'BHaO ; and 

 the dihydrate Col2'2H20 can exist together in presence of water. The colour of 

 the mixture varies from a yellowish to a greenish brown, and passes through every 

 shade of colour producible by mixing light passed through the red and green 

 solutions in different proportions, or that of a ray of white light which has passed 

 successively through the red and the green solutions. 



Some of the hydrated salts are not always easily prepared ; for instance, the 

 copper compounds CuCl2"2H20, and CuBra-SHjO, the reason being that the most 

 stable forms of these salts in solution are the molecules CuCl2"H20, CuCL, and 

 CuBrj. 



The cupric bromide CuBrj-SHjO undergoes a sort of black efHorescence at or 

 about 15° C. in air not artificially dried, losing thereby d per cent, of its water of 

 crystallization. But this salt deliquesces in moist air, and forms a solution of the 

 anhydrous compound which is black, or in very thin layers intensely dark madder 

 brown in colour. The peculiarity of this salt is that the balance in favour of 

 efflorescence or deliquescence depends upon very small differences in the pressure 

 of aqueous vapour in air at the ordinary temperature, and therefore corresponding 

 in barometric pressure of not more than a few millimetres of mercury ; so that on 

 each side of a mean condition, we have the formation of two differently constituted 

 molecules, which may be obtained in the form of solutions. Temperature, of 

 course, plays a part in such changes, and complicates the conditions under which 

 the substance crystallizes as a pentahydrate. For instance, the greenish golden 

 crystals deliquesced to black or brown solution at 15° ; but the same salt crystallized 

 * Mecanique Chimique, vol. ii., p. 160. 



