296 Hartley — The Action of Heat on the Absorption Spectra and 



Absorption Spectra modified by the Solubilittj of Salts. 



Tlie solubilit}' 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 coj^per, with theii' 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 is 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 alread}^ 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 tyjDical of 

 that of other dehydrating liquids, and may therefore be explained more fully. 

 Concentrated hydrochloric acid contains the anh3-drous acid HCl, the heat evolved 

 in the hydration of which exceeds that which accompanies the formation of 

 CoCl2*6H20 from the dihydrate CoCl2"2H20, or from tlie anliydrous chloride 

 C0CI2. 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 dih3'drate CuCl2"2H20, to that of the 

 anhydrous chloride CUCI2. If the solution is gi-een, it is a change from the mono- 

 hydrate CuCla'HjO to the anhydrous state within the solution.* Examples with 

 other chlorides could be cited, but these examples suffice to show the action of 



* Eugel shows that cupric chloride combines with HCl. The formula of one salt is CuClj'SHjO'HCl., 

 CR. 106, p. 273, 1888. 



