REVIEW OF EARLIER WORK. 163 



by Babo and Gladstone, and seems to be of the same general opinion as to 

 their cause that there is a change in the magnitude of the hydration. 



Bersch* in 1867 proposed a new view to account for the color changes in 

 the case of cobalt salts. He recognized tha^ when CoCl 2 .6H 2 is heated, it 

 yields two well-defined compounds, CoCl2.4H 2 and CoCl 2 .2H 2 0, the former 

 being bluish-red and the latter violet in color. But he thought that he 

 established the fact that the compound CoCl2.6H 2 may become blue with- 

 out loss of water, and concluded, therefore, that there are two modifications 

 of the compound CoCl 2 .6H 2 the one red and the other blue, and that the 

 first change in color on heating is due to the transformation of the red into 

 the blue modification. 



Tichbornef called attention to an interesting and important fact which 

 is stated in his own words: 



"It is easy to portend that although impossible at ordinary atmospheric pressure, and 

 in an ordinary aqueous solution, to dissociate the water, it is only necessary to boil such a 

 solution under sufficient amount of pressure to obtain the thermoanalytic point. This 

 was demonstrated by the following experiment: A weak solution of chloride of cobalt 

 was sealed up in a glass tube, two-thirds of the capacity of which was empty. On boiling 

 the liquid in this tube the solution gradually passes with the increment of heat through 

 all the shades of purple, until the contents ultimately become pure blue. Thus, in this 

 aqueous solution we have attained by extraordinary pressure, the temperature necessary 

 for the separation of the water. The change of color may be easily observed as it occurs 

 in the capillary tube, by holding any white material at the back of the tube and opposite 

 the experimenter." 



It is obvious that this observation is of importance in connection with 

 the theory as to the cause of these color changes. 



Attention was again called to this same fact by ClowesJ about two years 

 later. 



The spectrum of cobalt chloride was early studied, especially by Vogel. 

 He pointed out that the red aqueous solution of cobalt chloride shows only 

 one broad band in the green, and this lies between F and D. In stronger 

 solutions there is absorption of the violet, blue, and green, the less refrangi- 

 ble boundary of this region alone being transparent. When the solution of 

 cobalt chloride is evaporated nearly to dryness it becomes blue. The absorp- 

 tion of the blue and green completely disappears, and two sharp bands appear 

 in the red and orange. Vogel also studied the spectrum of the blue alcoholic 

 solution, and pointed out certain relations between it and the spectrum 

 of the blue aqueous solution. He observed, however, that the absorption 

 bands are closer to the red in the aqueous than in the alcoholic solution. 



* Sitzungsber, Wien-Akad., n, 56, 726 (1867). 



fChem. News, 25, 133 (1872). 



% Ibid., 29, 161 (1874). 



Ber. d. deutsch. chem. Gesell., 11, 913 (1878). 



