Chemical Constitution of Saline Solutions. 307 



Cohalt Bromide, Cvl'QiyQIIoO. — lOgrs. were drenched with 5 c.c. of water at 15°-0, 

 which caused the temperature to fall immediately, and it gradually sank to 8°'5. 



Some of the solid remained undissolved, as was the case with the co^iper salt. 

 Of the solution, 5 c.c. were mixed with 5 c.c. of water, both liquids being at 15° -2 

 The thermometer rose after the mixing to 16°'5. 



Nickel Bromide, NiBr2"6H20. — 10 grs. of the large green crystals were drenched 

 with 6 c.c. of water at 14°-5. The temperature sank to 11°'0. A saturated solution 

 of the salt at 16°'3 was mixed with an equal volume or 5'0 c.c. of water at 16°"5, 

 when the temperature immediately rose to 18°*1. 



It is evident that in each of these experiments, the cold saturated solution of the 

 salt combined with a further quantity of water to form a more complex molecule 

 in the solution. The results are even more striking with cobalt iodide. In some 

 cases the actual compounds contained in tlie diluted solutions have been deter- 

 mined by the extent to which the freezing point is lowered (Riidorff);* also 

 from thermo-chemical measurements (Thomsen),"|" and from the vapour joressures 

 of the solutions (De Coppet).J 



VI. Conclusion. — Saturated solutions of hygroscopic and deliquescent salts combine 

 with water ivhen diluted to constitute molecides of more complex hydrated compounds m 

 such solution. 



VII. 



In reviewing the facts already recorded, we arrive at the following statement 

 which may be considered as a law : — 



Wlien a saturated solution of a coloured salt undergoes a great change of colour upon 

 dilution, or any remarkable change in its absorption spectrum due to the same cause, 

 the dilution is always accompanied by a considerable evolution of heat. 



It is impossible to believe that the rise of temperature, upon dilution, is not 

 caused by the formation of a complex hydrated molecule in the solution, because 

 the change in colour is precisely that which occurs in the formation of the crystal- 

 line hydrates, and is the reverse of that of heat. 



Arguments have been advanced by Ostwald,§ and sujjported by Ewan,|| from 

 observations on the absorption spectra of very dilute solutions in favour of the 

 electrolytic dissociation theory propounded by Arrhenius. "It is a necessary 

 consequence of this theory that^ the absorption spectrum of an electrolyte, which 



* Poggendorfi's Annalen, vol. 114, p. 63, 1861, and vol. 145, p. 599, 1872. 

 f Thennocliemisclie Untersuchungen, vol. iii., 1883. 



J Ann , de Chemie et de Phys., vol. xxv., 4tli Series, p. 502 ; also vol. xxvi., p. 98, 1872. 

 § Zeitschr. physik. Chem., vol. 9, p. 579, 1892. || Proc. Roy. Soc, vol. 57. p. 117, 1894. 



^ Nernst's " Theoretical Chemistry." English Edition, pp. 335-337, 1895. 



2 U 2 



