TiCHBORNE ~ On Action of Heat upon Hydrated Salts. 249 
tlie alcohol should contain no water, and it is desirable to rectify 
from anhydrous sulphate of copper, or chloride of calcium. The 
following instructive illustrations may be shown with chloride of 
cobalt.* 
a — A blue alcoholic solution, obtained in the above manner, is 
placed in a deep beaker, and water is cautiously poured down the 
side of the vessel. Two layers will be produced, of two different 
colours, and will remain in this condition for some considerable time, 
by virtue of their different gravities. The upper layer will be blue, 
and contain the anhydrous salt ; the lower will be pink, and contain 
the hydrated salt, rendered so by the direct addition of water. 
^ — A beaker of the alcoholic solution which has been hydrated 
by the addition of water is peculiarly sensitive to heat. If it is 
gradually heated upon a water bath, it passes, as the temperature 
rises, through all the shades of pink and purple, until a pure blue ia 
produced, giving the same absorption spectrum as that obtained from 
the anhydrous salt. The thermanalytic point is so lowered by the 
alcohol present that the water of hydration of the cobalt salt is 
gradually but perfectly dissociated. 
If we now submerge this beaker half-way into a freezing mixture, 
we, in a short time, obtain similar chromatic and chemical results as 
in the first instance; but in this case the phenomena are brought about 
by different means. In the beaker we have two layers, exhibiting the 
upper or blue one, containing the anhydrous salt, the water being 
present, although dissociated. 
7 — It was easy to portend that, although impossible, at ordinary 
atmospheric pressure, and in an ordinary aqueous solution, to dis- 
sociate the water, it is only necessary to boil such a solution under a 
sufficient amount of pressure to obtain the thermanalytic point. 
This was demonstrated by the following experiments A weak 
solution of cobalt was sealed up in a glass tube, §rds of the capacity of 
which was empty. On boiling the liquid in this tube, the solution 
gradually passed through all the shades of purple, until the contents 
ultimately became of a pure blue. Thus, in this aqueous solution we 
had obtained, by extraordinary pressure, the temperature necessary 
for the separation of the water. 
Chloride of copperf gives a beautiful blue solution in water, which 
is characteristic of its hydrate. It gives a brownish yellow salt in 
the anhydrous state ; its solution in alcohol gives a greenish yellow ; 
or, in pure aether, which has been acidulated, a brownish yellow appear- 
ance. The beautiful blue solution of the neutral salt, when heated in 
a sealed tube to a high temperature, becomes gradually green, yellow, 
and ultimately a dark brown, and nearly opaque liquid. As it cools, 
it gradually associates the water of hydration, and passes again through 
all these shades ; but when cool it becomes a little opalescent, from 
* The composition of the hydrated salt is C0CI2, 6H2O, Marignec. 
t Cu CI2, 2 H2O, Graham. 
R. I. A. PROC— VOL. I., SEH, II., SCIENCE. 2 K 
