mass of gas per unit of volume--
will become the same on both sides. 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. De Vries,
moreover, by his remarks on living cells, extended the results which
Pfeffer had applied to one case only--that is, to the one that he had
been able to examine experimentally.

Such are the essential facts of osmosis. We may seek to interpret them
and to thoroughly examine the mechanism of the phenomenon; but it must
be acknowledged that as regards this point, physicists are not
entirely in accord. In the opinion of Professor Nernst, the
permeability of semi-permeable membranes is simply due to differences
of solubility in one of the substances of the membrane itself. Other
physicists think it attributable, either to the difference in the
dimensions of the molecules, of which some might pass through the
pores of the membrane and others be stopped by their relative size, or
to these molecules' greater or less mobility. For others, again, it is
the capillary phenomena which here act a preponderating part.

This last idea is already an old one: Jager, More, and Professor
Traube have all endeavoured to show that the direction and speed of
osmosis are determined by differences in the surface-tensions; and
recent experiments, especially those of Batelli, seem to prove