296 HartLtey—The Action of Heat on the Absorption Spectra and 
Absorption Spectra modified by the Solubility of Salts. 
The solubility of salts in water modifies the absorption spectra of solutions ; 
the less soluble salts of any metal form the paler solutions, as might be expected. 
This is evident on comparing the solutions of the sulphates of similar constitution, 
such as the sulphates of cobalt, nickel, and copper, with their corresponding 
chlorides and bromides. The sulphate of each is the least soluble compound. 
On the Action of Different Solvents. 
In so far as the solvent affects the chemical constitution of saline solutions, 
it 1s necessary to refer to two classes of solvents, those which are dehydrating 
agents and those which are not. ‘To the former category belong alcohol, 
calcium chloride, hydrochloric acid, and, in a modified degree, glycerine. These 
substances produce, in various degrees, the same general effect as rise of tem- 
perature, that is to say, they convert red cobalt compounds into more or less 
blue liquids, and grass-green copper solutions into brown and opaque liquids. 
The variations, caused by the difference in solubility of the compounds in the 
different liquids, have been already mentioned; but it was found that variations 
in solubility were not involved in modifying the absorption of light in these 
particular instances; for when an alcoholic solution or a hydrochloric acid 
solution was diluted with more alcohol or acid, the colour of the liquid altered to 
no further extent than would have been the case upon the addition of a colourless 
solvent having no chemical action upon the solution. It did not change the 
character of the absorption spectrum, but only the depth of tint of the solution. 
The action of hydrochloric acid upon solutions of hydrated salts is typical of 
that of other dehydrating liquids, and may therefore be explained more fully. 
Concentrated hydrochloric acid contains the anhydrous acid HCl, the heat evolved 
in the hydration of which exceeds that which accompanies the formation of 
CoCl,6H,0 from the dihydrate CoCl,2H,O, or from the anhydrous chloride 
CoCl,. Hence when solutions of the two are mixed, there is a change from red to 
blue caused by the partial or total dehydration of the cobalt chloride in solution. 
The same action occurs with cupric chloride, the change of colour is from green to 
brown, and the change of constitution of the solution, if, in the first instance, it 
is blue, is from that of a solution of the dihydrate CuCl,2H,0, to that of the 
anhydrous chloride CuCl. If the solution is green, it is a change from the mono- 
hydrate CuCl,"H,O to the anhydrous state within the solution.* Examples with 
other chlorides could be cited, but these examples suffice to show the action of 
* Engel shows that cupric chloride combines with HCl. The formula of one salt is CuCl,-3H,0-HC1., 
C.R. 106, p. 273, 1888. 
