166 HYDRATES IN AQUEOUS SOLUTION. 



He objects to the dehydration theory, that the blue color is due to the pres- 

 ence of the anhydrous salt, since this salt is light blue, while the solutions 

 are much deeper blue, and, indeed, often indigo blue. He objects to the 

 theory of Wyrouboff, since the monohydrate is violet and never blue, and 

 the solutions are often blue. 



Engel does not believe that any general theory can be advanced to account 

 for the changes in color which cobalt chloride undergoes. He thinks that 

 each special case must be dealt with separately. He is of the opinion that 

 the blue color is due to the presence of compounds which cobalt chloride 

 is capable of forming with various substances. Some of these compounds 

 are not blue notably the one formed with zinc chloride. Mercuric chloride, 

 stannous chloride, and the chloride of antimony behave like the chloride 

 of zinc. 



Engel supports his theory that the blue color is due to the presence of 

 double chlorides, by stating that he has obtained a double chloride of cal- 

 cium and cobalt, and calls attention to the fact that Chassevent* has pre- 

 pared a blue double chloride of cobalt and lithium, having the composition 

 CoCl2.LiC1.3H 2 O, which is probably analogous to CoCl2.HC1.3H 2 0, the com- 

 pound formed with hydrochloric acid. The double copper salt is, however, 

 red. Engel thinks that the blue color produced in a saturated solution 

 by rise in temperature, is not due solely to the presence of the monohydrate, 

 which is violet, but to the formation of a compound with hydrochloric acid, 

 the hydrochloric acid being liberated as the result of the action of water on 

 cobalt chloride. 



Wyrouboff* answers a number of the objections advanced by Engel 

 against the dehydration theory, especially concerning the color of the mono- 

 hydrate. It is largely a question of physical state, whether we are dealing 

 with transmitted or reflected light. Le Chatelier* does not doubt the exist- 

 ence of blue double salts chlorhydrates, alcoholates, etc. but thinks the 

 color changes can easily be explained on the dehydration theory. He points 

 out that the formation of the acid chloride would involve also the formation 

 of the oxychloride, which would be precipitated, and this is not the case. 



Le Chatelierf shows that both red and blue cobalt chloride act on calcium 

 carbonate, and that this and other lines of evidence adduced by Engel against 

 the hydration theory are not valid. 



EtardJ studied the changes in the absorption spectra of cobalt chloride 

 with change in temperature, and found with rise in temperature the dis- 

 appearance of certain bands and the appearance of others. This work is 



*Bull. Soc. Chim., [3] 6, 3 (1891). 

 t/Wd., [3] 6, 84 (1891). 

 JCompt. rend., 120, 1057 (1895). 



