516 On the Vapour-pressures of Water from Salt-Solutions. 



measure of the physical work done in effecting the change of 

 state of the salt — that is, in overcoming the cohesion of the 

 salt. If this is so, then the cohesion of the four salts under 

 consideration increases from NaCl to KN0 3 . 



Now the effect of concentration is to markedly increase the 

 restraining effect of the salt in the case of NaCl, to a less extent 

 with KC1 ; and, on the contrary, to diminish it with NaN0 3 , 

 and still more so with KN0 3 . But the cohesion is small with 

 NaCl and increases up to KN0 3 . Is it not reasonable to sup- 

 pose that, in the case of a salt with small cohesion, concentra- 

 tion has but little effect on the adhesion ; and that, on the 

 contrary, when the cohesion is very large, the effect of con- 

 centration is to slowly diminish the adhesion in each case till 

 the saturation-point is reached ; and salts with cohesion inter- 

 mediate between the extremes above will have an intermediate 

 effect on the vapour-presure of the water ? 



We have next to consider the effect of rise of temperature 

 on the restraining effect of the salt. This we have seen is a 

 diminishing one with NaCl, nil with KC1, and an increase in 

 the case of NaN0 3 and KN0 3 . This, I submit, is fully in 

 agreement with the above, and also with the observed solubi- 

 lities ; for though an increase of solubility implies an increase 

 of the difference between adhesion and the sum of the cohe- 

 sions, still that is in few cases due to an actual increase of the 

 value of the adhesion, but to the decrease of the cohesions 

 exceeding the decrease of the adhesion. Now in a solution of 

 a salt of constant strength, rise of temperature is necessarily 

 attended by a decrease in the value of all the three forces. If 

 the cohesion be diminished a little more than the adhesion the 

 salt will be a little more soluble, and the effect on the restrain- 

 ing effect of the salt will be to diminish it ; for though the 

 comparative value of the adhesion be increased, the absolute 

 value is diminished. On the other hand, if the cohesion be 

 very large and be largely affected by rise of temperature, the 

 result will be to increase the comparative value of the adhesion ; 

 and in strong solution to actually increase the absolute value, 

 as is shown by the increase in the restraining effect of KN0 3 , 

 especially in strong solutions. 



Thus it appears that this theory of solution is able to explain 

 the very varied phenomena of vapour-pressures of water from 

 salt-solutions, as it has explained other phenomena of solu- 

 tion. It remains to be proved whether this explanation is 

 correct. 



