PRESIDENTIAL ADDEESS — SECTION B. 95 



in right of which it proceeds to dissolve. This pressure is 

 analogous to the vapour-pressure of a volatile body in space, 

 the space being represented by the solvent ; and the process 

 of solution is analogous to that of vaporisation. As the con- 

 centration increases, the osmotic pressure of the dissolved por- 

 tion increases and tends to become equal to that of the 

 inidissolved portion, just as, during vaporisation in a closed 

 space, the pressure of the accumulating vapour tends to be- 

 come equal to the vapour-pressure of the liquid. But if there 

 be enough water present the whole of the solid will go into 

 solution, just as the whole of a volatile body will volatilise if 

 the available space be sufficient. Such a solution may be 

 exactly saturated or unsaturated. With excess of the solvent 

 it will be unsaturated ; and the dissolved matter will then be 

 in a state comparable to that of an unsaturated vapour, for 

 its osmotic pressure will be less than the possible maximum 

 corresponding to the temperature. On the other hand, if there 

 be not excess of water present during the process of solution, 

 a condition of equilibrium will be arrived at when the osmotic 

 pressure of the dissolved portion becomes equal to the pressure 

 of the undissolved portion, just as equilibrium will be esta- 

 blished between the volatile substance and its vapour if the 

 space be insufficient for complete volatilisation. In such a 

 case we get a saturated solution in presence of undissolved 

 solid, just as we may have a saturated vapour in presence of 

 its own liquid or solid. 



So far we have supposed the temperature to be stationary ; 

 but it may be raised. Now, a rise of temperature will disturb 

 equilibrium in either case alike ; for osmotic pressure and 

 vapour-pressure are both increased by this means, and a re- 

 establishment of equilibrium necessitates increased solution or 

 vaporisation, as the case may be. 



Now, what will this constantly-increasing solubility with 

 rise of temperature eventually lead to? Will it lead to a 

 maximum of solubility at some definite temperature beyond 

 which increase becomes impossible ? Or will it go on in the 

 way it has begun, so that there will always be a definite, 

 though it may be a very great, solubility for every definite tem- 

 perature? Or will it lead to infinite solubility before infinite 

 temperature is attained ? One or other of these things must 

 happen, provided, of course, that chemical change does not 

 intervene. 



Well, let us be guided by the analogy that has hitherto 

 held good. Let us see what this leads us to, and afterwards 

 examine the available experimental evidence. We know that 

 a volatile liquid will at last reach a temperature at which it 

 becomes infinitely volatile — a temperature above which the 

 liquid cannot possibly exist in the presence of its own vapour, 



