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 by partial dehydration in 
solution. When a salt which is one of several hydrates is dissolved in water, the 
compound formed in the 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 the action of the dehydrating substance at the temperature of the 
solution. 
When a deliquescent salt forms a saturated solution with 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 
