Chemical Constitution of Saline Solutions. 309 



are not blue, nor nickel green, nor cobalt red ; neither are the respective solutions 

 blue, green, nor red. In the case of crystalline hydrates, with water as a solvent, 

 it depends upon the temperature whether the salt is simply dissolved or converted 

 into another substance, either by further hydration or b)^ partial dehydration in 

 solution. When a salt which is one of several hydrates is dissolved in water, the 

 compound formed in tlie solution is that which at the particular temperature and 

 degree of concentration of the solution has the greatest stability ; it may be 

 simply a homogeneous mixture of the original molecule with water, or of the 

 anhydrous molecule and water, or it may be some intermediate compound 

 according to the nature of the salt, but, in any case, it is that compound which has 

 the greatest stability according to the mass and temperature of the solvent. If 

 the solvent combines with water, the salt formed is that which has the greatest 

 stability under tbe action of the dehydrating substance at the temperature of the 

 solution. 



When a deliquescent salt forms a saturated solution witli water, the homo- 

 geneous mixture of the salt and water-molecules continues to absorb water as if 

 the solvent substance were not present, that is to say, with evolution of heat until 

 the saturation point has been attained. Slight variations in temperature affect 

 the hydration of the solution, so that it gains or loses with depression and rise of 

 temperature, respectively. Changes in colour and in absorption spectra, which 

 are characteristic of solutions of anhydrous compounds, or of different solid 

 hydrated salts, are also characteristic of aqueous solutions when placed under such 

 conditions as would lead to the formation of such compounds, whether hydrated 

 or anhydrous. For instance, when the temperature of the liquid is favourable to 

 dissociation, then the hydrated molecule is dissociated from more or less of that 

 water, which in the solid is called water of crystallization, and such dissociation 

 occurs at a lower temperature than when the salt is in the solid state. 



The action of heat on the absorption spectra of aqueous solutions of salts 

 which do not form hydrates, is not characteristic of any chemical change, but 

 merely of a purely physical phenomenon which may be explained by a greater 

 amplitude in the molecular and intra-molecular vibrations. The effect is similar 

 to that caused by concentrating the solutions. But if the absorption spectra of 

 aqueous solutions containing hydrated salts undergo any marked alteration either 

 in the amount of light or in the wave-length of the rays absorbed, this is always 

 accompanied by a change in the composition of the molecule. The nature of the 

 chemical change is either a dissociation of water from the molecules of the salt or 

 a decomposition into a basic compound and free acid, which is a characteristic 

 change when solutions of the blue or purple chromium compounds are changed to 

 green upon raising the temperature of their solutions. In such cases, the recon- 

 version of the solution into its original molecular condition either does not take 



