Photophoresis

Abstract

THE ratio of the surface area to the volume of a spherical body varies inversely as its radius. This fact led F. Ehrenhaft (Ann. d. Phys., IvL, 81, 1918; cf. Phys. Zeit., xv., 608, 1914; and xviii., 352, 1917) to use small spherical particles of various substances, produced either by volatilisation or by burning an electric arc between electrodes of the substance in an inert atmosphere, in his examination of the forces exerted by stationary light radiation on matter. Such particles, suspended in argon or nitrogen, were introduced into a small observation chamber, which was strongly illuminated from the side, the observations being made with a microscope placed perpendicular to the illuminating pencil of light. Ehrenhaft concentrated the light from an arc—with exclusion of the ultra-violet and infra-red rays—to a small conical pencil, with a diameter of only i/io mm. at its narrowest part. The behaviour of the small spherical particles in and near the region of most intense illumination was studied.