Page:Elementary Text-book of Physics (Anthony, 1897).djvu/117

§ 89] a rise of temperature, which may be so great as to raise the solid to incandescence. Thus, for example, spongy platinum, placed in a mixture of oxygen and hydrogen, becomes so heated as to inflame it.

89. Free Diffusion of Liquids.—When two liquids which are miscible are so brought together in a common vessel that the heavier is at the bottom and the lighter rests upon it in a well-defined layer, it is found that after a time, even though no agitation occurs, they become uniformly mixed. Molecules of the heavier liquid make their way upwards through the lighter; while those of the lighter make their way downwards through the heavier, in apparent opposition to gravitation. Diffusion is the name which is employed to designate this phenomenon and others of a similar nature.

When one of the liquids is colored,—as, for example, solution of cupric sulphate,—while the other is colorless, the progress of the experiment may easily be watched and noted. When both liquids are colorless, small glass spheres, adjusted and sealed so as to have different but determinate specific gravities between those of the liquids employed, may be placed in the vessel used in the experiment, and will show by their positions the degree of diffusion which has occurred at any given time.

90. Coefficient of Diffusion.—Experiment shows that the amount of a salt in solution which at a given temperature passes, in unit time, through unit area of a horizontal surface, depends upon the nature of the salt and the rate of change of concentration at that surface,—that is, the quantity of a salt that passes a given horizontal plane in unit time is $$\kappa CA$$, where $$A$$ is the area, $$C$$ the rate of change of concentration, and $$\kappa$$ a coefficient that depends upon the nature of the substance. By rate of change of concentration is meant the difference in the quantities of salt in solution, measured in grams per cubic centimetre, at two horizontal planes one centimetre apart, supposing the concentration to diminish uniformly from one to the other. It is plain, that, if $$C$$ and $$A$$ in the above expression be each equal to unity, the quantity of salt passing in