It may be desirable to explain how this fundamental fact was recognised. It has long been known to the physiologist that certain membranes are _semi-permeable_—that is, they allow of the passage of certain liquids, and of substances in solution, to the exclusion of others. This phenomenon is termed _osmosis_, and is of great biological significance. It was first studied by plant-physiologists, notably by Traube and Pfeffer. Many such semi-permeable membranes can be formed artificially, but the most generally convenient is found to be one consisting of copper ferrocyanide deposited on the walls of a porous vessel. If a vessel so prepared be filled with a solution of sugar, and be then placed in water, the water is found to pass through the membrane, but the membrane is impermeable to the sugar. In consequence pressure, termed _osmotic pressure_, is found to occur within the pot, and may be measured by suitable means. These osmotic pressures may at times be very large: thus a 1 per cent. solution of sugar may exert a pressure of half an atmosphere, and in the case of a solution of potassium nitrate of the same concentration it may amount to a couple of atmospheres. Pfeffer determined the relation of the osmotic pressures to the concentration of solutions of these substances, measuring the pressures in centimetres of mercury by a manometer attached to the closed porous vessel. His results in the case of sugar were as follows: _Percentage _Pressure in cm. strength (C)._ of mercury (P)._ _P/C._ 1 53.5 53.5 2 101.6 50.8 4 208.2 52.1 6 307.5 51.3 It will be seen from these numbers that the ratio P/C is practically constant—that is, _the osmotic pressure varies directly as the concentration_. It was further found that the osmotic pressure exerted by a solution of uniform strength increases with the temperature.