Page:Elementary Text-book of Physics (Anthony, 1897).djvu/120

106 substances in solution. They may be called semi-permeable membranes.

Pfeffer's results may be described most easily by using as an example the case of solutions of cane-sugar and water. When the cell was filled with water containing sugar in solution, and immersed in pure water, the water began to enter the cell from without; the pressure in the cell, as indicated by the manometer, began at once to increase, and continued increasing for some time, until a rather large definite increase of pressure had been reached. This increase of pressure in the cell is called the osmotic pressure or solution pressure of the solution. The most important laws established by Pfeffer, de Vries, and others concerning the relations of osmotic pressure to the character of the solution and its circumstances are as follows, it being understood that the statements to be made refer to dilute solutions and to solutions which are not electrolytes (§ 279). Solutions which are not electrolytes may be called indifferent solutions (§ 385). For solutions which are electrolytes the statements need some modifications.

The osmotic pressure is independent of the nature of the solvent and of the character of the membrane, provided it is impervious to the substance dissolved.

The osmotic pressure is proportional to the concentration of the solution or to the quantity of the dissolved substance contained in unit volume. It increases as the temperature rises, and the relation between the increase of pressure and the rise of temperature is the same as that which obtains for gases (§ 211).

Weights of different substances which are proportional to the molecular weights of those substances contain equal numbers of molecules. Solutions formed by dissolving, in equal quantities of the solvent, masses of different substances proportional to the molecular weights of the substances, therefore contain equal numbers of molecules of these substances. They may be called equimolecular solutions. It is found that the osmotic pressure exerted by equimolecular solutions of different substances is the same. Solutions which exert equal osmotic pressures are called isotonic.