of Water from Salt- Solutions. 515 



reached at which the three forces are in equilibrium ; this is 

 the saturation-point for the temperature under consideration. 

 At the saturation-point as many salt molecules meet and 

 unite to form solid salt in unit time as are dissolved by the 

 action of the water on the still undissolved salt. 



The effect of temperature on each of these three forces is 

 to diminish each of them. It may be each to the same ex- 

 tent, or it may be by unequal amounts ; and thus a salt may 

 have its solubility unaltered, increased, or diminished by rise 

 of temperature. That is, according as the difference between 

 the adhesion and the sum of the cohesions at the higher tem- 

 perature is > , =, < the same difference at the lower tempera- 

 ture, so is the solubility at the higher temperature > , = , < than 

 the solubility at the lower temperature. But this is not all, 

 for the magnitude of these differences depends on the various 

 effects of temperature on the adhesion and the sum of the 

 cohesions; and this may be very different with different salts, 

 for though the cohesion of the water is affected by tempera- 

 ture always to the same extent, still the cohesion of the salt 

 and the adhesion are variables. With this preliminary ex- 

 planation, I will proceed at once to the explanation of the 

 effect of salts on the vapour-pressure of water according to 

 the above theory. 



The first point to be noted is that all four salts have, in 

 dilute solution, very nearly the same restraining effect : — 



w. 



2 



10 



NaCl. 



KC1. 



NaN0 3 . 



KN0 3 



4-25 



3-80 



4-25 



3'75 



5-04 



4 43 



3-67 



2-73 



but that in strong solutions it is very different. Now the 

 restraining effect of a salt can only be due to the adhesion of 

 the water and salt*; this, then, is very nearly a constant for 

 all four salts, and is still more nearly a constant for salts of 

 the same metal. Next, the heat of solution of the salts is as 

 folio wsj — 



NaCl. KC1. NaN0 3 . KN0 3 . 



-1180 -4400 -5200 -8500, 



still the same series before observed. Now, as pointed out in 

 another paper, if the restraining effect of salt be a constant, 

 or nearly so, then the heat of solution of various salts is a 



* It may be noted, in passing, that unless a salt molecule were capable 

 of disturbing the equilibrium of the whole of the water moleeules present, 

 it could have no effect on the vapour-pressure of the water. 



