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 molecules, whose probable dimensions are about one thousand times less, their probable velocity would be, as required by the kinetic theory, some hundreds of metres per second. In the case of objects we can only just see, the Brownian velocity is only a few thousandths of a millimetre per second. No doubt, concludes M. Gouy, the particles that show this velocity are really enormous when compared with true molecules. From this point of view the Brownian movement is but the first degree, and a magnified picture of the molecular vibrations assumed in the kinetic theory.

§ 3.

Migration of Material Particles.—In the Brownian movement we take into account only very small, isolated masses, small free fragments—i.e., material particles which are not hampered by their relations to neighbouring particles. Any one but a physicist might believe that in true solids endowed with cohesion and tenacity, in which the molecules were bound one to the other, in which form and volume are fixed, there could be no longer movements or changes. This is a mistake. Physics teaches us the contrary, and, in late years especially, has furnished us characteristic examples. There are real migrations of material particles throughout solid bodies—migrations of considerable extent. They are accomplished through the agency of diverse forces acting externally—pressures, thrusts, torsions; sometimes under the action of light, sometimes under the action of electricity, sometimes under the influence of forces of