Chemical Constitution of Saline Solutions. 303 
When a salt is put into water, the water may (1) 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) the temperature, 
and (2) 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 anhydrous 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, 
CuBr,5H,0; and cobalt iodide is another for the hexahydrate Col,6H;,0; and 
the dihydrate Col,"2H,O can exist together in presence of water. The colour of 
the mixture varies froma 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 CuCl,’2H,0, and CuBr,5H,0, the reason being that the most 
stable forms of these salts in solution are the molecules CuCl,"H,O, CuCl, and 
CuBr,. 
The cupric bromide CuBr,"5H,O undergoes a sort of black efflorescence at or 
about 15° C. in air not artificially dried, losing thereby 9 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 
* Mécanique Chimique, vol. ii., p. 160. 
