Molecular Thermodynamics. 615 



—at temperature T and pressure p. That is, by (31 ) and (33) 



=:jAT-i(i.-iW (34) 



where L cals. per gm. is absorbed in passing info phase (a), 



( )c.c./gm. is dilatation on passing into phase (a) 



\po a Po J 



and (/>!', etc., correspond to temperature T and pressure _£>. 



In Cryoscopic and Ebullioscopic data, Ap is zero. 



In Vapour pressure and Freezing pressure data, AT is 

 zero. 



(b) Osmotic Pressure data. Here the phases are artificially 

 separated, and maintained at different pressures, the "solvent' 

 being in the same molecular condition in both. 



Since m 0o and m are the same 



hno a z=—8n . 



For equilibrium of the two phases under different pressures 

 as indicated by the suffixes p and po, 



m = (<m + 1 ( log (1+tCi)+t<j>J \ fi ^ ei m } . 



\m /p \nioJp m I ° L O^iJ J P 



That is 



??i T 

 P 



m Q 



_^Ap 



" P oT> ( * 5) 



where P (or Ap) is (a small) "osmotic pressure/' Now 

 from the form of the expansion of log (l + #) it is clear that 

 log (1 + Xci) approximates to %ci when, and only when, 

 ^S^i is negligible compared with unity. When in addition 

 the general terms are negligible we get 



P H^ 



— rfi = ^'l 



or 



kPo 





