270 LIFE AND DEATH. 



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. The Internal Activity of Bodies. 



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 — migra- 

 tions 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 elec- 

 tricity, sometimes under the influence of forces of 



