Only then will equilibrium be
established; and, at that moment, an excess of pressure will naturally
be produced in that recipient which, at the commencement, contained
the gas with the smallest quantity of hydrogen.
This experiment enables us to anticipate what will happen in a liquid
medium with semi-permeable partitions. Between two recipients, one
containing pure water, the other, say, water with sugar in solution,
separated by one of these partitions, there will be produced merely a
movement of the pure towards the sugared water, and following this, an
increase of pressure on the side of the last. But this increase will
not be without limits. At a certain moment the pressure will cease to
increase and will remain at a fixed value which now has a given
direction. This is the osmotic pressure.
Pfeffer demonstrated that, for the same substance, the osmotic
pressure is proportional to the concentration, and consequently in
inverse ratio to the volume occupied by a similar mass of the solute.
He gave figures from which it was easy, as Professor Van t'Hoff found,
to draw the conclusion that, in a constant volume, the osmotic
pressure is proportional to the absolute temperature.
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