Page:Elementary Text-book of Physics (Anthony, 1897).djvu/121

§ 96] Solutions which are isotonic at one temperature are isotonic at all temperatures, or, the change of osmotic pressure with temperature is the same for all equimolecular solutions. The absolute value of the osmotic pressure is the same as the pressure which would be exerted by a gas contained in the same vessel as the solution, and having the same number of molecules as there are molecules of the substance dissolved.

It should be stated that these laws have not all been proved conclusively by experiment, but they are well established on theoretical grounds.

95. Dissociation.—The foregoing laws of osmotic pressure do not hold for all solutions. The deviations which appear in solutions which are not highly dilute are explained in the same way as the departure of highly-compressed gases from the similar laws of gaseous pressure, namely, by the absence of those simple conditions upon which only these laws are theoretically possible. The deviations of electrolytes from these laws, which are sometimes very great, have been explained by Arrhenius as the result of the separation of some or all of the molecules of the dissolved substance into their constituent portions or ions. This separation is called dissociation. The dissociation theory receives abundant support from the phenomena of electrolysis, and will be discussed in that connection (§ 285).

96. Laws of Diffusion of Gases.—Gases obey the same elementary laws of diffusion as liquids. The rate of diffusion varies inversely as the pressure, directly as the square of the absolute temperature, and inversely as the square root of the density of the gas. A gas diffuses through porous solids according to the same laws. An apparatus by which this may be conveniently illustrated consists of a porous cell, the open end of which is closed by a stopper, through which passes a long tube. This is placed in a vertical position, with the open end of the tube in a vessel of water. If, now, a bell-jar containing hydrogen be placed over the porous cell, hydrogen passes into the cell more rapidly than the air escapes from it: the pressure inside is increased, as is shown by