118 BUTLER ART. D 



dissolve the ice. Similarly the addition of a salt to water 

 causes a decrease in the pressure of water vapor which is in 

 equihbrium with the hquid at the same temperature. Both 

 phenomena show "that m (the potential for water in the liquid 

 mass) is diminished by the addition of the salt, when the tem- 

 perature and pressure are maintained constant. Now there 

 seems to be no a priori reason for supposing that the ratio of 

 this diminution of the potential for water to the quantity of the 

 salt which is added vanishes with this quantity. We should 

 rather expect that, for small quantities of the salt, an effect of 

 this kind would be proportional to its cause, i.e., that the differ- 

 ential coefficient in [211] would have a finite negative value for 

 an infinitesimal value of vi2. That this is the case with respect 

 to numerous watery solutions of salts is distinctly indicated by 

 the experiments of Wtillner* on the tension of the vapor yielded 

 by such solutions, and of Rlidorff f on the temperature at which 

 ice is formed in them; and unless we have experimental evidence 

 that cases are numerous in which the contrary is true, it seems 

 not unreasonable to assume, as a general law, that when nh has 

 the value zero and is incapable of negative values, the differ- 

 ential coefficient in [211] will have a finite negative value, and 

 that equation [212] will therefore hold true." We may observe 

 that the truth of this law has been confirmed by numerous 

 more exact experimental investigations. 



The change of mi caused by the addition of a small amount 

 drrh of S2 is evidently inversely proportional to the amount 

 (mi) of Si, so that we may write, in the limiting case, when 

 W2 = 0, 



P) = - -' (114) 



( 



where A' is positive and independent of mi. 

 Then, by (110), 



m2 



\dm-ijt. p. m, 



* Pogg. Ann., 103, 529 (1858); 105, 85 (1858); 110, 564 (1860). 

 t Pogg. Ann., 114, 63 (1861). 



