REVIEW OP EARLIER WORK. 167 



analogous to that of Becquerel,* which deals especially with the absorption 

 spectra of salts of didymium and uranium. 



As Etard points out, the appearance of the new bands with rise in tem- 

 perature shows that there is some new internal arrangement of the mole- 

 cules, as we should expect if there was a change in the hydration as the 

 temperature changes. As the nature of the molecules in the solution changes 

 new absorption bands will make their appearance. 



The most elaborate work, by far, that has been done on the absorption 

 spectra of cobalt salts is that of Hartley. f This is but part of an extensive 

 investigation on the absorption spectra of a number of colored salts. This 

 work includes also the chloride and bromide of copper. 



The spectra of an aqueous solution of cobalt chloride, at temperatures 

 ranging from 23 to 93, were photographed, and the bands also measured. 

 Hartley concludes that his photographs of these spectra of the solution 

 at different temperatures show that molecules of water are split off from 

 the cobalt chloride with rise in temperature. The absorption bands widen 

 in the red with rise in temperature, showing that the molecule is becoming 

 simpler, and vibrates in resonance with a larger number of waves. 



Hartley worked also with solutions of cobalt chloride in absolute alcohol 

 at different temperatures, and concluded that rise in temperature did not 

 affect the alcoholic solutions. He also worked with solutions of cobalt 

 chloride in glycerol; with cobalt chloride mixed with hydrochloric acid, and 

 with cobalt chloride to which a solution of calcium chloride had been added. 

 Work similar to the above was done with cobalt bromide and iodide. 



The effect of rise in temperature on the absorption spectra of a number 

 of salts of chromium was also studied. 



Hartley concludes that the effect of rise in temperature, in general, on 

 the absorption spectra of solutions of salts is as follows: If the salts are 

 anhydrous or not dehydrated at 100, or if they do not change color when 

 dehydrated, they do not change their absorption spectra when heated. 

 Hydrated salts change their spectra with change in temperature, and this 

 change is usually very nearly the same as that effected by dehydrating agents. 



OstwaldJ concludes that the red color of solutions of cobalt salts, in gen- 

 eral, is due to the cobalt ion. He points out that the hexahydrate easily 

 loses water and forms lower hydrates which are blue in color. If the con- 

 centrated aqueous solutions are heated, they turn blue. This same color 

 change is very easily produced by adding to the solution of cobalt chloride 

 chlorine ions. Thus, the presence of hydrochloric acid or sodium chloride 



*Ann. Chim. Phys., [6] 14, 170 and 257 (1888). 

 tTrans. Roy. Soc. Dublin, n, 7, 253 (1900). 

 t Grundlinien d. anorgan. Cheni., 620. 



