304 Hartley — 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 temj^erature 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. Conclusion. — Cri/stallized 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. Conclusion. — The most stable compound which can exist in a saturated solution 

 at 16° or 20° is 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 crystallisa- 

 tion when the molecule enters into solution. 



The Colour Relations of Solid Salts to those of the same Substances in Solution. 



The molecule CuCl2'2H20 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°. 



