ON VAPOUB PEESSUBES AND EEFEACTIVE INDICES OF SALT SOLUTIONS. 207 



fact that the saturated solutions of pairs of salts which do not crystallise 

 together are unaffected by excess of either salt. 



IV. Eorpansion of Salt Solutions. 



The results described in the previous Report have been completely 

 examined, and will soon be pubhshed. The following may be added to 

 the conclusions already arrived at : — 



The effect of heat on the volume of a solution of a salt depends on 

 the solubility of the salt as a function of the temperature. If the solu- 

 bility be little affected by temperature then the volume curve approaches 

 more nearly to a straight line than when the solubility is largely de- 

 pendent on temperature. 



In the former case the effect of heat is simpler than in the latter. In 

 the one the solution is practically of the same strength throughout. In 

 the other the rise of temperature is attended not only by expansion, biit 

 also by what is practically dilution of the solution. Thus it is at present 

 impossible to trace out a further connection between solubility and rate 

 of expansion of a salt in solution. 



V. Water of Crystallisation. 



An examination of the evidence derivable from the results of thermo- 

 chemical investigations, and also a comparison of the molecular volumes 

 of dissolved salts, lead to the conclusion that no part of the water in a 

 solution of a hydrated salt can be said to be in a different relation to the 

 salt from that of the remainder of the water. In other words, water of 

 crystallisation cannot be recognised in solution either by thermal or 

 volume changes — it is indistinguishable from the rest of the water. The 

 argument based on colour changes of solutions of C0CI2, &c., does not 

 affect the above, for it is not contended that the salt is anhydrous in the 

 Bame sense as it is when dried at 150° C. 



Second Report of the Committee, consisting of Professors Ramsay, 

 TiLDEN, Maeshall, and W. L. Goodwin (Secretary), appointed 

 for the purpose of investigating certain Physical Constants of 

 Solution, especially the Expansion of Saline Solutions. 



Graham, in a series of interesting experiments, has shown that saline 

 solutions absorb water- vapour from a saturated atmosphere (' Edin. Journ. 

 of Science,' xvi. 1828, pp. 326-335 ; also Schweigger, ' Journ.' liii. 1828, 

 pp. 249-264). This process he called inva'poration. His experiments 

 were made by enclosing, in a tin canister containing water, glass basins 

 in which were equal weights of (generally) saturated solutions. After a 

 few days the canister was opened, the dishes weighed, and the gain of 

 water by invaporation thus determined. The relative rates of invaporation 

 thus became approximately known. But these rates estimated in this 

 way are influenced by the rate of diffusion of water- vapour in air, and by 

 the rates of diffusion of the salts in water. The latter especially must be 

 taken into account in interpreting Graham's results. A salt with strong 



