holars most closely identified with the classic tradition. It
may perhaps not be without use to examine here the principles of
Professor Van t'Hoff's theory.

Sec. 2. OSMOSIS

Osmosis, or diffusion through a septum, is a phenomenon which has been
known for some time. The discovery of it is attributed to the Abbe
Nollet, who is supposed to have observed it in 1748, during some
"researches on liquids in ebullition." A classic experiment by
Dutrochet, effected about 1830, makes this phenomenon clear. Into pure
water is plunged the lower part of a vertical tube containing pure
alcohol, open at the top and closed at the bottom by a membrane, such
as a pig's bladder, without any visible perforation. In a very short
time it will be found, by means of an areometer for instance, that the
water outside contains alcohol, while the alcohol of the tube, pure at
first, is now diluted. Two currents have therefore passed through the
membrane, one of water from the outside to the inside, and one of
alcohol in the converse direction. It is also noted that a difference
in the levels has occurred, and that the liquid in the tube now rises
to a considerable height. It must therefore be admitted that the flow
of the water has been more rapid than that of the alcohol. At the
commencement, the water must have penetrated into the tube much more
rapidly than the alcohol left it. Hence the difference in the levels,
and, consequently, a difference of pressure on the two faces of the
membrane. This difference goes on increasing, reaches a maximum, then
diminishes, and vanishes when the diffusion is complete, final
equilibrium being then attained.

The phenomenon is evidently connected with diffusion. If water is very
carefully poured on to alcohol, the two layers, separate at first,
mingle by degrees till a homogeneous substance is obtained. The
bladder seems not to have prevented this diffusion from taking place,
but it seems to have shown itself more permeable to water than to
alcohol. May it not therefore be supposed that there must exist
dividing walls in which this difference of permeability becomes
greater and greater, which would be permeable to the solvent and
absolutely impermeable to the solute? If this be so, the phenomena of
these _semi-permeable_ walls, as they are termed, can be observed in
particularly simple conditions.

The answer to this question has been furnished by biologists, at which
we cannot be surprised. The phenome