Reaction before Complete Equilibrium, 91 



started, &c. The constant usually decreases with the increase 

 of concentration. The velocity constants of ice separation 

 and of ice melting are very great also in the case of cane- 

 sugar ; and we can very well assume that the constant remains 

 the same in dilute solutions. 



Some very interesting experiments have been made by 

 Stephan. Though he did not arrive at the law of velocity of 

 evaporation itself, he made the very interesting observation 

 that the velocity with which the same quantity of liquid 

 evaporates at constant temperature and surface varies with 

 the pressure of the air or gas resting upon the liquid : — 



here V is the velocity of evaporation, jh is the partial pres- 

 sure of the vapour, P— p t the partial pressure of the air or 

 gas. The equation must evidently hold good for the velocity 

 of condensation of the saturated vapour to a liquid when the 

 pressure of the air or gas resting upon the liquid varies. 

 Quite an analogous law is also to be expected for the velocity 

 of separation of salts from supersaturated solutions, or for 

 the solution of a solid in a liquid if an indifferent substance 

 be dissolved in the solvent. In the latter case P — p 1 will be 

 the concentration or osmotic pressure of the indifferent sub- 

 stance and p x the solution pressure of the solid at the given 

 temperature. The constant C, as is evident, must vary with 

 the solvent. Moreover, we can say that Stephan's law and 

 others analogous exist only because the velocity constant 

 varies with the solvent of the gas or of the dissolved sub- 

 stance, and that pure water, c. </.. and water containing an 

 indifferent substance in solution are no longer the same solvent. 

 (This also explains the deviations from Henry's law of 

 absorption observed when indifferent substances are dissolved 

 in the solvent.) 



The analogy between the evaporation of a liquid or the 

 sublimation of a solid, on the one hand, and the solution of a 

 solid salt, on the other, allows us to establish the following 

 relation, one not very well admitting experimental proof: — 

 The velocity of condensation of oversaturated vapour to a liquid 

 or a solid is, at a constant surface of the liquid or of the solid, 

 directly proportional to the degree of supersaturation. The 

 velocity of evaporation of a liquid and ike velocity of sublimation 

 of a solid, are, at a constant surface of the liquid or of the solid r 

 directly proportional to the remoteness ot the system from 

 the point of equilibrium, or from the point of saturation. If 

 the surface varies during the reaction, the velocity of reaction 



