Physical Notes

Abstract

IN liquids small particles often show dancing motions under the microscope, and similar motions have been attributed to dust-particles in air, and accounted for by the shock of molecules with the particles. In a recent paper treating fully of the movements of very minute bodies (Münch. Ber., 1879, P. 389) Herr Nageli calculates from data of the mechanical theory of gases as to the weight and number and collisions of molecules, the velocity of the smallest fungus-particles in the air that can be perceived with the best microscopes, supposing a nitrogen or oxygen molecule to drive against them. It is, at the most, as much as the velocity of the hour-hand of a watch, since these fungi are 300 million times heavier than a nitrogen or oxygen molecule. The ordinary motes would move 50 million times slower than the hour-hand of a watch. Numbers of the same magnitude are obtained for movements of small particles in liquids. In both cases a summation of the shocks of different molecules is not admissible, as the movements are equably distributed in all directions. Herr Nägeli therefore disputes the dancing motion of solar dust-particles, and attributes the Brownian molecular motion to forces active between the surface-molecules of the liquid and the small particles; but he does not say how he conceives of this action.