96 REPORT— 1891. 



no matter how great the pressure may be. At this tempera- 

 ture equihbrium of pressure between the hquid and its vapour 

 becomes impossible, and above this point the substance can 

 exist only as a gas. This is the critical temperature. And so 

 it seems to me that, if we carry our analogy to its logical con- 

 clusion, we may expect for every substance and its solvent a 

 definite temperature above which equilibrium of osmotic 

 pressure between undissolved and dissolved substance is im- 

 possible, — a temperature above which the substance cannot 

 exist in presence of its own solution, or in other words a tem- 

 perature of infinite solubility. This may be spoken of as the 

 critical solution-temperature. 



But a little consideration shows that in one particular we 

 have been somewhat inexact in the pursuance of our analogy. 

 For we have compared the solution of a solid body to the 

 vaporisation of a volatile liquid. We can, however, do better 

 than this, for volatile solid bodies are not wanting. It is to these, 

 then, that we must look in the first instance. Now, a volatile 

 solid (such as camphor or iodine) will not reach its critical point 

 without having first melted at some lower temperature ; and a 

 similar change should be exhibited in the solution process. 

 At some definite temperature, below that of infinite solubility, 

 we may expect the solid to melt. This solution melting -2Joint 

 will not be identical with, but lower than, the true melting- 

 point of the solid ; and for the following reason : No case is 

 known, and probably no case exists, of two liquids one of which 

 dissolves in the other and yet cannot dissolve any of it in 

 return. Therefore there will be formed, by melting, not the 

 pure liquid substance, but a solution of the solvent in the liquid 

 substance. Hence the actual melting- or freezing-point will be 

 lower than the true one, in right of the laws of which I have 

 spoken when discussing Eaoult's methods in the earlier part of 

 this address. 



From this solution melting-point upwards, we shall then 

 have to deal with two liquid layers, each containing both sub- 

 stance A and solvent B, but the one being mostly substance A 

 and the other mostly solvent B. These may be spoken of as 

 the A layer and the B layer. As temperature rises the pro- 

 portion of A will decrease in the A layer and increase in the 

 B layer ; and every gramme of A will occupy an increasing 

 solution-volume in the A layer (B being absorbed there) and a 

 decreasing solution-volume in the B layer. At each tempera- 

 ture the osmotic pressure of A in the two layers must be equal. 

 The whole course of affairs, as thus conceived, now admits of 

 the closest comparison with the changes which accompany 

 gradual rise of temperature in the case of a volatile liquid and 

 its saturated vapour. The liquid is like the substance A in the 

 A layer; the vapour (which is the same matter in another 



