418 On Supersaturated Saline Solutions. 



10-atom salt, small hard granular crystals of the anhydrous 

 salt are formed as the solution becomes concentrated. 



LoweFs speculations as to the formation of the 7-atoin salt 

 need not be gone into here ; nor need I repeat my own expe- 

 riments, conducted in 1867, which led me to the conclusion 

 that it is the anhydrous salt that is in solution ; but I do wish 

 to repeat the following consideration by which the same con- 

 clusion was arrived at. 



In the experiments for determining the elasticity of steam 

 from pure water, the elasticity is diminished for any given 

 temperature if a small portion of a salt, such as soda, soluble 

 in water, and not capable of rising in vapour with it, be 

 allowed to ascend to the top of the mercurial column ; the 

 column rises, thereby indicating a diminished elasticity of 

 steam. The adhesion of the soda to the water tends to restrain 

 the water from evaporating, and this tendency is a measurable 

 force and here measured ; for it partly balances the tension 

 of the water or its tendency to emit steam, and it thus makes 

 the steam-emitting tension of a solution of soda measurably 

 less than that of pure water at the same temperature. 



Now it is clear that if, instead of soda, Glauber's salt be 

 used in experiments of this kind, there will be a smaller 

 amount of diminution in the elasticity of the steam if the salt 

 in solution retain its water of crystallization than if it en- 

 tered into solution in the anhydrous form. In a valuable 

 series of experiments by Wiillner (Pogg. Ann. ciii. p. 529, 

 and ex. p. 564) it appears to have been established for solu- 

 tions of various strengths (such as 10, 20, 30, &c. per cent, of 

 Glauber's salt in pure water), and at various temperatures 

 from 10° to 100° C, that the diminution in the elasticity of the 

 steam is proportional to the quantity of dry salt in solution ; 

 and, further, that at the point of maximum solubility of this 

 salt no molecular change takes place, or it would have im- 

 pressed itself on the curve which represents the elasticity of 

 the steam. The general conclusion for the salts tried is that 

 the action of efflorescent salts is expressed in terms of the dry 

 Bait, while for deliquescent salts it is in terms of the hydrated 



Dr. NicoPs papers are so admirable, that I trust he will 

 continue his researches into other matters connected with this 

 difficult subject. 



Highgate, April 15, 1886. 



