368 ' Mr. W. W. J. Nicol on the 



aqueous vapour correspond. This is the usual method of ex- 

 pressing the action of a salt on the vapour-pressure of water — 

 that is, it raises the boiling-point so many degrees. 



It will be seen that the number of degrees that the boiling- 

 point of water is raised is a quantity that increases with the 

 temperature in all cases where the solubility also increases, but 

 diminishes in a more or less marked manner when the solu- 

 bility decreases with the rise of temperature. All this can be 

 readily explained. It is only to be expected that the larger 

 number of molecules of salt present should have a greater 

 influence on the water-molecules ; but this explanation fails 

 when the case of KN0 3 of the same strength is examined. 

 Here the quantity of salt remained the same, and yet the 

 increase of rise of boiling-point is very well marked. That 

 the strength of the solution was the same at the end as at the 

 beginning of the experiment was proved by weighing the tube 

 before and after : the loss of water and salt adhering to the 

 thermometer did not amount to '08 per cent. In this case, 

 then, it appears that the restraining effect of the salt on the 

 water increases with the rise of temperature. 



Before attempting to explain this, I wish to draw attention 

 to a much more convenient mode of expressing the action of 

 salt on water with regard to its vapour-pressure. If at a tem- 

 perature t the vapour-pressure of water is pt, and at the same 

 temperature the pressure of water- vapour from a salt-solution 

 is p f t , then the pressure of pure water is diminished by the 

 salt p t ~ p't millim. Now as the value of p increases faster 

 than the temperature, in order to compare pt—p't with pt>— p'# 

 we must divide by pt and pt' respectively, giving us 



1- W and 1_ ~ pp &c -> 



numbers expressing the effect of the salt at those temperatures. 

 If the salt-solution have the strength n molecules of salt to the 



P 



100 molecules of water, then expresses the effect of each 



molecule of salt at that particular temperature. 



In Table III. are given the values of ( 1 — p ) x 10,000 and 

 the solubilities at the corresponding temperatures, with the 



P 



values of also multiplied by 10,000. It is evident that 



the effect of rise of temperature on the attraction of salt for 



