SOLUTION. 335 SOLUTION. when a gas is eveiilj- 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 volume of pure water be placed over our diluted solution of sugar, and diffusion upward against gravity, as well as in all otiier 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 cither 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 emplo}' 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 cjlindrical vessel with a tight-fitting piston just touching the solution. If the piston is made of a solid imprrmeable material, then 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 semi-per- meable material, through which the water, but 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. The best artificial semi-permeable material thus far discovered, especially well adapted for separating water from dissolved sugar, is a mem- brane of ferrocyanide of copper, formed by the action of potassium ferrocyanide upon copper sulphate. Pfeffer, who was the first to employ this substance for measuring the pressure of substances in solution, proceeded as follows: He filled a porous clay cylinder with a solution of copper sulphate and immersed it in a solutioir of potassium ferrocyanide: the two solutions, pene- trating into the clay from the opposite sides, yielded a precipitate of copper ferrocyanide where they met within the walls of the cylinder. the walls serving to impart to the precipitated membrane considerable mechanical resistance. The cylinder was now filled with a solution of sugar, its upper end was tightly closed with a lid bearing an ordinary mercury manometer, and the apparatus was jjlaced in pure water so that the level of the latter was i)recisely the same as that of the solution within. To understand the phenomenon that followed, inuigine a cylin- drical vessel AI'CD in which, say. air has been compressed within the vohnue EFCD, while the space ABFE is empty; if we relieve the piston EF, it will be driven up bj' the expansive power of the air until it is stopped by AB or by some other resistance ; if, instead, we A 17=1 f=n B hold up the e.ylinder in the air by the handle, the expansive power of the compressed air will cause the entire volume ABCD to move over, the result being, again, a larger space occupied by the air. Precisely analogous phenomena would be observed if EFCD were filled with a solution of sugar were pure water, while EF were T D and ABFE a semi- permeable membrane; Either the piston would move upward or the entire liquid volume (pure water plus solution ) would move in the direc- tion of the dissolved sugar; in either case the cause would be the expansive power of the sugar and the result an increase of the volume occupied by it, i.e. an addition of pure water to the solu- tion. As a matter of fact, in Pfeffer's apparatus, the semi-permeable walls being fixed, the expan- sive power of the dissolved sugar caused pure water to enter the clay cylinder. The increasing amount of liquid naturally caused an increasing compression of the air within the cylinder, and finally a point was reached when the expansive power of the sugar was no longer capable of over- coming the resistance of the air, the latter having grown precisely equal to it. Then equilibrium ensued, the mercury manometer showing the pres- sure of the air within the cylinder, and hence the equal of that pressure — the 'osmotic pressure' of the sugar in solution. Similar experiments showed: (1) That the osmotic pressure of sugar and other substances in dilute solutions is pro- portional to the concentration, i.e. inversely pro- portional to the volume of the solution: (2) that the osmotic pressure of sugar and other substances in dilute solution is proportional to the absolute temperature (i.e. the centigrade temperature plus 273 degrees) ; (3) that the osmotic pressure of substances in dilute solution is equal to the pressure that the solute would exert if removed from the solution, vaporized, and inclosed within an empty volume equal to that of the solution, at a temperature equal to that of the solution. In brief, the laws of gases, viz, the law of Boyle and Mariotte, the law of Charles and Gay- Lussac, and Avogadro's rule, hold good in the case of dilute solutions as they do in the case of gases. Further experiments have shown, be- sides, that the osmotic pressure in solutions is the same no matter what the solvent. The importance of these results will be evident to those who realize that the science of chemisti-y is based on the laws of the gaseous state, Avo- gadro's rule, which embodies those laws, being the only sure guide in finding those comparable
