Diffusion

Diffusion. The particles of all material bodies, except such as may be totally devoid of heat, are in rapid motion. In the case of solid bodies the excursions of any one particle are limited to a small space; but in fluids a particle may move more or less freely throughout the whole space occupied. This intermixture of molecules may occur also when different fluids are placed in contact with each other, but it may be prevented by the existence of tension at the common surface (see SURFACE-TENSION and CAPILLARITY). When it does occur, the fluids are said to diffuse into each other.