Prof. J. H. Poynting on Osmotic Pressure. 299 



be altered in the same ratio and the vapour- tension would 

 be unaltered. But we are supposing that they are inactive 

 for evaporation only and that their a molecules of combined 

 solvent are still active for condensation. So that in the 

 solution there are only N— an active for evaporation, while 

 there are still N active for condensation. Hence the vapour- 

 tension is reduced in the ratio — ^ — . Or if -cr and «■' are 

 the solvent and solution vapour-tensions, 

 m' _ N — an 



and 



ct — ijs 1 an 



~~*T ~ N * 



If each salt molecule takes one solvent molecule, so that 

 a = l, we have 



nr — -to 7 n 



~~^r = n' 



which is the usual result deduced for dilute solutions from 

 the van't Hoff value of the osmotic pressure. We may, of 

 course, work backwards from this result, and the work may 

 be put in the following form : — 



If P is the pressure in the solution necessary to restore its 

 mobility to that of the solvent, i. e, to increase it in the ratio 



or 



and 



If M is the molecular weight of the solvent 



OT '(l + 



-or p J 



:*S, 



-57 — Tz' 



P<7_ 



'usp 



n 



'US 



N 



P = 



wasp 







M 



