8o GENERAL CONDITIONS OF EQUILIBRIUM, chap.
Ai + A^^ aT r21n V + In'^i"! = 0,
L 2^1 irJ
Van't Hoff (J) has shown that when Henry's law — that the quantity of gas dissolved per unit volume is proportional to the gas-pressure— obtains, the gas has the same molecular magnitude in the two conditions (gaseous and dissolved). We might have a solution of the gas in another solvent {e,g. ether) in contact with the liquid (water) instead of the gas itself. If in the first process of the foregoing series we make use of a semi-permeable membrane, which allows the ether but not the gas to pass through, it can be shown that —
^1 TTl p\ '
^ and ^1 denote the osmotic pressures of the dissolved gas in the ether in vessels A and B,
Provided that a substance — whether capable of existence in the gaseous state or not — possesses the same molectdar weight in two solvents, the osmotic pressures, and conse- quently also the concentrations of the substance in the two liquids, must stand to each other in a constant ratio.
Distribution Law. — If an aqueous solution of succinic acid be shaken with ether, part of the dissolved substance passes into the ether. If this be carried out with aqueous solutions of dififerent concentrations, the amount of succinic acid which passes into solution in unit volume of ether must increase with rising concentration of the aqueous solution. Experimentally it has been found that the following law of distribution holds good : when equilibrium is established the concentration of the ethereal solution is proportional to that of the aqueous solution. The following table contains the results obtained at 15° by Berthelot and Jungileisch (4); ci and C2 denote the weights in grams of succinic acid in 10 c.c. of water and ether respectively. At higher concentrations
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