OSMOTIC PRESSURE. It may be seen from the above that a theory of solutions does not yet exist. Some of the most important questions with regard to solutions remain unanswered and the known facts are mostly uncorrelated: in brief, the subject is largely not yet rationalized.
In one of its phases. however, the subject of solutions has, within recent years, received a development which must he counted among the most brilliant scientific achievements of our time. The achievement in question is based on the most characteristic property of solutions, viz, the capacity of the 'solute' (i.e. the dissolved sub stance) to diffuse within the solution until the concentration of the latter is the same at all its points. Let an aqueous' solution of sugar, for in stance, be placed at the bottom of a vessel, and let some pure water be introduced over it, can tionsly, so as not to disturb the solution; the result will he that the sugar will gradually dif fuse upward, and after a certain length of time the liquid will have a perfectly uniform com position throughout. Now, to cause this motion of the sugar upward. against gravity, there must obviously be some force. An analogous case that readily suggests itself to the mind is that of gases. A gas, too, will flow upward, and. like a substance in solution, will distribute itself evenly within an available volume. Of course, when a gas is evenly distributed within a vessel, it still exercises pressure on the walls, while in the case of a substance in solution, once diffusion is over, there would seem to be no evidence of the existence of a pressure. Yet there, too, a pressure must exist ; for let a new CNN' volume of pure water be placed over our diluted solution of sugar, and diffusion upward against gravity, as well as in all other directions from points of higher to points of lower concentration, will recommence.
All this suggests that, in general, the proper ties of matter in a highly dilute state (i.e. when a small mass occupies a large volume) may be the same whether the dilute state is that of a gas or that of a substance in solution. For in
either of those states matter possesses the most important characteristic of gases, viz. the ca pacity for expanding indefinitely. The problem therefore arises. to ascertain whether the laws of the interrelation of pressure, volume, and temperature of substances in solution are not similar to, or identical with, the corresponding laws of gases—a problem that can be solved only by experimental inquiry. The volume and tem perature are evidently those of the solution and can be easily measured. So the solution of the problem depends on a method for measuring the pressure of the solute. To measure this directly, it is obviously necessary to employ an apparatus by means of which it would be possible to exert pressure upon the solute without at the same time exerting pressure upon the solvent—in other words, an apparatus for separating the solvent and the solute. Such an apparatus would show the resistance offered by the solute alone and would thus furnish a measure of its pressure. Let. for instance, an aqueous solution of sugar be placed in a cylindrical vessel with a tight-fitting piston just touching the solution. If the piston is made of a solid impermeable material. that external pressure upon it will be resisted by the solution as a whole, most of the resistance being of course offered by the water, which is highly incompressible. If, on the other hand. the piston is made of some ordinary, permeable, filtering, material, then external pressure upon it will scarcely be resisted at all, the solution as a whole passing through it. Evidently, to answer our purpose, the piston must be made of a se i- per meable material, through which the water, hut not the sugar dissolved in it. could pass freely. By means of such a piston alone could we compress the sugar without compressing the water and thus ascertain the resisting pressure of sugar within the volume of the solution, as we might ascertain the pressure of a gas within an ordinary vessel.