318 Hartley — On the Conditions of Equilibrium of Deliquescent and Hygroscopic 



temperature is about 57°'75, even when the pressure of aqueous vapour is 0'3, or 

 0"-i inches of mercury. 



This represents altogether I2H2O, or the increase over the initial weight of 

 the salt is 30'4 per cent., which represents absorbed water. The salt is wholly 

 liquid. Before this, however, there is a wavering between eight and nine mole- 

 cules of water. 



Cohalt Bromide, CoBra'BHaO. — The fluctuations occur with a maximum of 61° 

 and a minimum of 47°, or a mean of 54°, but the difference amounts to ± 0*3 on 

 the total weight of 159-4 grs. Thus, for instance, with a tension of aqueous 

 vapour of 0-425, a maximum of 63 , and a minimum of 60°, equal to a mean 

 temperature of 61°'5, we obtain a total weight of 159-98 grs., or very nearly 

 60 per cent, over the initial weight of the salt. In these circumstances the salt is 

 wholly liquid. 



Nickel Bromide, NiBro-OHjO. — With a maximum temperature of 56°-5, and a 

 minimum of 41°" 3, equivalent to a mean of 45°, slight variations in weight occur 

 very similar to those observed in the case of cobalt bromide, but the excess over 

 the initial weight of the salt is 56-02 per cent, on a total of 156-02 grs. This 

 salt is wholly liquid. 



Nickel Iodide, Nil2*7H20.* — This salt increased in weight more rapidly than 

 any of the others, more especially at first, and showed no fluctuation until it 

 became converted into a salt with 23 molecules of water. 



The Absorption of Light by Solutions of Haloid Salts of Copper, Nickel, 



AND Cobalt. 



Tiie more nearly the copper and nickel salts approach in composition the 

 anhydrous state, it is observed that the greater is the absorption of light by the 

 molecules. The effect of diluting the concentrated green solution ef cuprio 

 chloride produces a blue colour, resembling that of the crystallized hydrate, 

 CuCl2'2H20 ; and of the brown solution of the cupric bromide, a green and after- 

 wards a blue solution, much less absorbent for the rays of white light than the 

 original solution. The colour of the deliquescent concentrated solution of cupric 

 chloride is, however, still grass-green, and that of the deliquesced cupric bromide 

 is brown. These facts, at first sight, appear to be out of harmony with the 

 degree of hydration of the salts as determined by the weight of water absorbed 

 in the foregoing record of exjjeriments, but the conditions existing in the 

 solutions must be taken into account along with the intensity of colour of 

 the different compounds. The solution of CuCl2-2H20 in a minimum of water 

 is much dai'ker than the solid salt, as also is the solid salt darker when very 



*For this formula, see (Sci. Trans. Roy. Dub. Soc, vol. vii., ser. 11., 1900, p. 265. 



