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



The Illustrations. 



A word of explanation is necessary with respect to the curves. 



As the weighings of the salts on different occasions were carried out to a 

 degree of accuracy, represented by one-tenth of a milligramme, and as the initial 

 weight of the salt was represented as 100 parts, and was calculated to two 

 decimal places, it was found that a curve to represent either of these values 

 would have been unmanageable. Accordingly, it was decided that the ordinate 

 numbers should represent parts per 1000 of the weight in excess of the original 

 salt, and that the abscissae should be the days of the month on which the 

 weighings were made. We may thus consider zero as 1000, and the increase in 

 weight extends up to nearly 700 in excess of this. The whole of the curve has 

 not been given, but only the principal portions on three separate sheets. Thus 

 the beginning of the curve is not seen, but is indicated. 



To avoid complications, the temperature curves and the curves representing 

 the tension of aqueous vapour have been omitted. 



The following particulars are of interest regarding these compounds : — 



Cupric Chloride^ CuCl2'2H20. — Notwithstanding an increase in vapour pressure, 

 a rise of temperature caused a decrease in weight as soon as the mean temperature 

 had risen to 52° F., the maximum being 59°-0, and the minimum 45°-5. 



But a maximum of 64°*5, and a minimum of 51°, the pressure of aqueous 

 vapour being 0"305 inches of mercury, the reduction in weight was almost 

 down to the normal condition of the solid, or within TO jDer cent, of the initial 

 weight of the salt. In other words, not only had the salt ceased to be either 

 deliquescent or hygroscopic, but it had also lost the water it had previously gained. 



This accounts for the fact that, day after day, for many successive weeks, 

 some of this salt, contained in a porcelain dish, was always a green liquid in the 

 morning and a mass of blue crystals in the afternoon. It was freely exposed on 

 a laboratory table about three feet from a window and twenty feet from a - fire- 

 stove. The temperature of the room, therefore, gradually rose, and at night 

 sank again to a temperature at which the salt became deliquescent. 



Cupric Bromide^ CuBra, Anhydrous. — This is a steel-grey substance, with 

 brilliant metallic lustre. The behaviour of the compound is singular, for it is 

 strongly hygroscopic without being markedly deliquescent, and its hygroscopicity 

 is to a large extent independent of temperature, provided the atmosphere is well 

 charged with aqueous vapour. In comparison with the chloride, it shows a much 

 greater number of fluctuations over the same range of temperature and the same 

 conditions of humidity. 



Three different conditions of hydration, and at least two different hydrates. 



