Nevertheless, in the case of gases, there exists an excellent example of a semi-permeable wall, and a partition of platinum brought to a higher than red heat is, as shown by M. Villard in some ingenious experiments, completely impermeable to air, and very permeable, on the contrary, to hydrogen.  It can also be experimentally demonstrated that on taking two recipients separated by such a partition, and both containing nitrogen mixed with varying proportions of hydrogen, the last-named gas will pass through the partition in such a way that the concentration—­that is to say, the 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 that osmosis establishes itself in the way which best equalizes the surface-tensions of the liquids on both sides of the partition.  Solutions possessing the same surface-tension, though not in molecular equilibrium, would thus be always in osmotic equilibrium.  We must not conceal from ourselves that this result would be in contradiction with the kinetic theory.