REVIEW OF EARLIER WOR#. 165 



In 1891 a number of papers appeared bearing upon the problem under 

 discussion. Etard* pointed out the importance of studying simultaneously 

 the color changes and solubility curve. He worked with cobalt chloride and 

 cobalt iodide, and showed that at those temperatures corresponding to the for- 

 mation of a new hydrate the solubility curve changed direction, being no 

 longer a straight line. The new hydrate would have different solubility 

 from the old, and this was clearly shown by the solubility curves. He thus 

 proved the existence of the following hydrates: CoI 2 .6H 2 0, CoI 2 .4H 2 and 

 CoI 2 .2HoO; CoCl 2 .6H 2 O and CoCl 2 .2H 2 O. 



Potilitzinf came back to the problem of the change in the color of cobalt 

 chloride, having published a paper seven years earlier on this same subject. 

 He points out that when crystals of CoCl2.6H 2 melt, they form a deep violet- 

 colored liquid, which becomes more and more deeply blue. He also points 

 out that hydrating substances, in general, produce the blue color. He 

 concludes that these changes in color are due entirely to changes in the 

 hydration of the cobalt chloride. 



CharpyJ studied the vapor-tension of solutions of cobalt chloride, and 

 plotted the results as curves. He found that the curve was made up of 

 two well-defined, rectilinear portions. The two portions of the curve met 

 at 75. Above this temperature the solutions were blue in color. This 

 agrees in a way with the results found by Etard. 



As Charpy points out, his results can be interpreted in either of two ways. 

 There is either a change in the hydration of the salt with change in tempera- 

 ture, or there is a change in the state of molecular aggregation. From these 

 results it is impossible to decide between the above two possibilities. 



Wyrouboff calls attention to the fact that the change in the color of 

 solutions of cobalt chloride with rise in temperature, can not be due to the 

 presence of the anhydrous salt, since this combines with water with the 

 greatest avidity, a large amount of heat being at the same time liberated. 

 He showed that the change in color was due to the formation of the mono- 

 hydrate CoCl 2 .H 2 0, which is formed in larger and larger quantities the 

 more elevated the temperature. This violet-colored hydrate is stable, not 

 losing its molecule of water until a temperature of 140 is reached, when it 

 passes over into the pale blue, anhydrous salt. 



Engel|| does not accept any of the theories that have been advanced to 

 account for the color changes that take place in cobalt chloride. He reviews 

 briefly our knowledge of the hydrates of cobalt chloride, and then takes 

 up the theories that have been proposed to account for the color changes. 



*Compt. rend., 113, 699 (1891). 

 fBull. Soc. Chim., [3] 6, 264 (1891). 

 JCompt. rend., 113, 794 (1901). 

 Bull. Soc. Chim., [3] 5, 460 (1901). 

 ||/Wd. f [3] 6, 239 (1891). 



