it to be under the pressure of its own vapour, the quantity to be 
calculated is the same as the thermodynamic potential of its saturated 
vapour, 1. e. 
of 
foe + Peoex. “coer. a ik (EL) 
Vcoex. 
where we denote by the index covx. that the quantity must be taken 
on the line of coexistence. Now is on account of the assumed 
validity of the gaslaws 
7 
a) 
ae =F Peoex. Ucoex. = MRT Upeoex./p, + MRT 
Voer. 
If we equate the expression obtained here with that of the 
preceding §, then #(7), MRT and MRT logp, neutralise each other 
on both sides. What is left we may write in this way : 
db Bt pe(1—2) Ope 
(Po — Pe) vo & — |= — MRT log ————— + a (ve, — Ve, ) + 
da Peoex. De 
da / 1 1 db ( 1 1 
WE Se cian Oe. ae lee ees 
Now ve, and v, can never differ much. If the osmotic pressure 
of an aqueous solution amounts e.g. to 1000 Atm., these volumes 
differ only a few percents. In the two last terms, which themselves 
can only be correction terms, we may therefore put 7, =v in any 
case, so that those terms vanish. Further we may neglect v, by the 
side of vy, and write MRT logp. for vo. Our equation becomes then: 
MRT |, Pe (1—2) Mogpe 
(SoS SS ba St 
OT db vu Poes. da 
Vo — & — 
i da 
The remaining v, may, of course, not be replaced by ve, first 
because this expression occurs here in the principal term and then 
because the substitution of 7, for v, would of course be more felt 
in a term of the order 1/v—é/ than in 1/v. But in any case, when 
we have really to do with osmotic pressures, the pressure will never 
be so large that we could not compute v, with the aid of the coeffi- 
cient of compressibility of the saturated liquid without any difficulty. 
§ 8. The quantity pop, which we have found, is not identical 
with the osmotic pressure; the latter is rather po—peoer., but the 
transition of one quantity to the other is without any difficulty. If 
