162 HYDRATES IN AQUEOUS SOLUTION. 



The fact that dehydrating agents produce the change in color from pur- 

 plish-red to blue, and, further, that the same color change is effected by rais- 

 ing the temperature of the solution, early suggested the view that the change 

 in color from reddish to blue was due to a loss of water on the part of the 

 salt in solution. 



Babo* observed that cobalt chloride in concentrated solution was colored 

 blue by absolute alcohol at ordinary temperatures. At more elevated tem- 

 peratures a few drops of alcohol are sufficient to produce the blue color. A 

 solution of calcium chloride or magnesium chloride changes the color of 

 cobalt chloride to blue very readily, especially at the boiling-points of the 

 solutions. A saturated solution of sodium chloride will also produce the 

 blue color at elevated temperatures. 



A concentrated solution of zinc chloride, on the other hand, gives only 

 a red color with cobalt chloride, even when the mixture is warm. Babo 

 thinks that this is due to the formation of a double salt. He concludes 

 that whenever there is a transformation from the reddish to the blue color, 

 there is a dehydration of the cobalt salt. 



Gladstone! in his well-known paper, "On the Use of the Prism in Qual- 

 itative Analysis," takes up the question here under discussion. In con- 

 nection with salts of cobalt he says: 



"We are accustomed to speak of blue and red salts of cobalt, but this difference depends 

 on the state of hydration, and the prism reveals a,n analogy otherwise unsuspected 

 between the two colors." 



He points out that the salts of cobalt, in general, when anhydrous are 

 blue, and that the hydrated salts or aqueous solutions are red. The only 

 exception seems to be the sulphocyanate, whose saturated aqueous solution 

 is blue. The dilute aqueous solution of even this salt is, however, red. In 

 his subsequent investigations with the loscope, he comes essentially to the 

 same conclusions as those pointed out above. $ He brought within the 

 scope of this investigation a large number of colored compounds. Among 

 the substances where the character of the color changes with the dilution, 

 are the three salts upon which special stress has been laid in this investi- 

 gation cobalt chloride, copper chloride, and copper bromide. 



In 1859 Schiff published a paper on the "Effect of Rise in Temperature 

 on the Intensity of the Color of Solutions," in which such questions as 

 those now under consideration were discussed. He added a number of 

 cases of color changes with change of temperature, to those already observed 



*Ber. Uber d. Verhandl. d. Gesell. f. Beford. d. Naturw. zu Freiburg, i. B. 1857, No. 

 17, 283. Jahresber, 1857, 72. 



fJourn. Chem. Soc. (Lond.), 10, 79 (1859). 

 I Ibid., 11, 36 (1?59). 

 Lieb. Ann., 110, 203 (1859), 



