Page:Elementary Text-book of Physics (Anthony, 1897).djvu/406

393 glass; he also showed that it could pass through other substances with varying degrees of facility. Among the effects ascribed by Lenard to this discharge were the production of fluorescence in many fluorescent substances, the production of photographic action in ordinary photographic dry-plates, and the penetration of the discharge through bodies by an amount dependent upon their densities, it being less as the densities are greater. The discharge was deflected when brought into a magnetic field.

In 1895 Röntgen discovered that effects in some degree similar to those investigated by Lenard could be obtained from any highly exhausted vacuum tube. The results of his researches and of those of many other physicists who have investigated the same action may be described as follows: Wherever the cathode discharge falls upon certain substances, the most important of which, as yet known, are platinum and glass, an action is set up known as the Röntgen radiance. This radiance excites fluorescence in many fluorescent substances and acts upon the photographic plate. It proceeds in straight lines and its intensity varies inversely with the square of the distance; it is not affected by the presence of a magnetic field; in these respects it apparently differs from the action investigated by Lenard. It penetrates all substances and is partly obstructed by all substances, the obstruction being greater as the density of the substance is greater. It is apparently capable of true reflection to a very small degree. No indubitable evidence has as yet been given that it can be refracted, or that it exhibits the phenomena of interference, diffraction, or polarization. When it falls upon an electrified body the charge on the body gradually disappears, the effect being to render the air or other gas surrounding the body a conductor.

No satisfactory theory of the Röntgen radiance can as yet be given. It has been variously ascribed to the mechanical movement of the molecules of the residual gas in the tube in which it originates or of the walls of the tube, to transverse vibrations in the ether of a wave length much shorter than those of the shortest waves of light hitherto known, and to longitudinal vibrations in the ether.