254 



The Earl of Berkeley on Solubility and 



XX. Solubility and Super solubility prom the Osmotic 

 Standpoint. By the Earl of Berkeley, F.R.S* 



(1) TTITHERTO solubilities and supersolubilities have 

 XI been discussed from the point of view of phase 

 rule solubility curves. I propose here to indicate another 

 method, that of osmotic equilibria, which seems to throw 

 additional light on the subject ; and at the same time I will 

 outline a hypothetical explanation of some of the observed 

 phenomena. 



(2) I will preface my remarks by drawing attention to 

 the fact that every binary mixture has, at a given temperature, 

 two osmotic pressures, one which is realizable by means of a 

 membrane permeable to the solvent, and the other, which 

 will be called the " conjugate " osmotic pressure, realizable 

 by a membrane permeable to the solute. Over and above 

 these, there are an infinite number of osmotic pressures 

 which merely depend on the standard conditions of pressure 

 adopted : thus, there is one set definable by the condition 

 that the pressure on the solvent remains constant, and 

 another set definable by the condition that the pressure on 

 the solution remains constant ; and farther, both these sets 

 give different osmotic pressures according to the magnitude 

 of the constant pressure. 



(3) Unless otherwise stated, the temperature will be 

 assumed constant, and the phrase " osmotic pressure " will 

 refer to solutions under a constant pressure p ; it will mean 

 the algebraic difference of pressure between p and the 

 pressure q (whether positive or negative) which has to be 

 put on the solvent to keep it in osmotic equilibrium with 

 the solution ; similarly with the conjugate osmotic pressure. 

 The conditions required are realized in fig. 1, where the 



Fig. 1. 



_R 



POftE 

 SOLUTE 



7 



SoLUl 



PUff£ 

 SOLVENT 



r> 



P£f?M£/)BLE TO P£ffMEABL£ TO 

 SOLUTL SOLVENT 



osmotic pressure is the difference between p and gi, while 

 the conjugate is that between p and q 2 ; the osmotic 

 pressures for all concentrations being measured with the 

 same pressure p on the solutions. 



* Communicated by the Author. 



