304 Harttey—The Action of Heat on the Absorption Spectra and 
out in the golden prisms when the solution was cooled to 7°, the colour of the 
solution remaining unaltered. 
A solution of cupric bromide, which had been evaporated gently by the aid of 
heat, but still at a low temperature, yielded no crystals at all when cooled from 
time to time out of contact with air to a temperature as low as 0°5° C. After 
repeated trials, it was found that the solution had a specific gravity of 1:8, and it 
seemed to be quite uncrystallizable. It then deposited only the black anhydrous 
salt, with a fine steely lustre on the faces of the crystals. When the salt so 
obtained was dried by keeping it for a time over oil of vitriol, it was transferred 
to a beaker glass, and exposed to air saturated with aqueous vapour at 15° to 17°, 
but generally at the lower temperature. It then lost its fine steely metallic lustre, 
swelled up, and became black like charcoal, while at the same time it gained 
in weight. During the night it became encrusted with the greenish, golden, 
prismatic crystals; and the minimum thermometer registered a temperature of 8°. 
These crystals disappeared when the surrounding air had been no warmer than 
15°; and they were again reformed, on one occasion in profusion, when the 
temperature had been so low as 3°5°, and the air saturated with water vapour. 
Similar results were observed with crystals of cupric chloride, which were pale 
blue at 15°, and a deep grass-green solution at 8°, recrystallizing to the blue salt 
at 15°, and again deliquescing to the deep green solution at the lower temperature, 
the air being saturated with moisture at the time. 
IV. Conctusion.— Crystallized hydrated salts, dissolved in a minimum of water 
at 20°C., undergo dissociation by rise of temperature. The extent of the dissociation 
may proceed as far as complete dehydration of the compound, so that more or less of the 
anhydrous salt may be formed in the solution. 
V. Conctusion.—The most stable compound which can exist in a saturated solution 
at 16° or 20° ds not always of the same composition as the crystalline solid at the same 
temperature, since the solid may undergo partial dissociation from its water of erystalliza- 
tion when the molecule enters into solution. 
The Colour Relations of Solid Salts to those of the same Substances in Solution. 
The molecule CuCl,'2H,O is a pale blue salt. (Seep. 302.) When dissolved 
in an equal weight of water at 16°C., it forms a grass-green solution, darker than 
the solid substance. This may be observed with the deliquescing crystals. When 
five parts of water are added to this, the result is a clear blue liquid. The pale 
blue crystals of this salt spread over the bottom of a large porcelain dish, standing 
about three feet from a large window, and about twenty-five feet from a fire, were 
observed day by day for several weeks to be a green solution in the morning, and 
blue crystals in the afternoon, the variations in temperature being from 8° to 15°. 
