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 phenomena of osmosis are naturally of the first importance in the action of organisms, and for a long time have attracted the attention of naturalists.

De Vries imagined that the contractions noticed in the protoplasm of cells placed in saline solutions were due to a phenomenon of osmosis, and, upon examining more closely certain peculiarities of cell life, various scholars have demonstrated that living cells are enclosed in membranes permeable to certain substances and entirely impermeable to others.