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2 GENERAL AND PHYSICO-CHEMICAL.
solution of potassium ferrocyanide. Thereby the drop of copper sulphate
is surrounded by a membrane of copper ferrocyanide, which is imper-
vious to copper sulphate as well as to potassium ferrocyanide, but allows
water to pass. The drops retain their blue color in the yellow solution
but increase in volume, due to the taking up of water, until the tension of
the membrane prevents the further increase in size. If the difference in
concentration of the two solutions is great enough, the membrane is
ruptured by the pressure.
In order to give the copper-ferrocyanide membrane a greater rigidity,
Pfeffer has suggested forming the precipitate on a porous, rigid wall. 1
For this purpose he makes use of a small, porous earthenware cell which,
after careful cleaning, is treated with copper sulphate and potassium
ferrocyanide so that the membrane is precipitated on the inner wall
of the cell. The membrane thus obtained is impervious to the cane-
sugar. If the cell is filled with a cane-sugar solution and then placed
in pure water, no sugar leaves the cell, while water passes into the cell,
and this continues until the opposite pressure produced prevents the
further passage of water. If the cell is completely closed and in. con-
nection with a manometer, then on the establishment of an equilibrium
the manometer indicates the force with which the inclosed solution
attracts water.
As the sugar is attracted with the same force by the water as the water
is by the sugar and also as the sugar cannot pass through the membrane
therefore the sugar exerts a pressure upon the membrane equal to the
pressure indicated by the manometer. This pressure is called the
osmotic pressure of the enclosed solution. For dilute cane-sugar solutions
Pfeffer’s determinations show that the osmotic pressure is approx-
imately proportional to the concentration and slowly rises with the
temperature.
Experiments with other semipermeable membranes have also been
carried out by de Vries, and these will be discussed on page 5. De
Vries’ experiments have led to the following result: Solutions of analo-
gously constructed bodies having the same molecular concentration give the
same osmotic pressure.
Van’t Hoff first called attention to the analogy which exists between
the laws of osmotic pressure of a dissolved substance and of gases,2
namely, that the osmotic pressure is proportional (or inversely propor-
tional to the volume of the solution) to the concentration, and corre-
sponds completely with Boyle-Mariotte’s law on the relation between
the volume and pressure of gases. Also, that equimolecular solutions
1
Osmotische Untersuchungen, Leipzig, 1877.
2
Zeitschr. f. physik. Chem., 1, 481 (1887).
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