In 1895 Sir William Ramsay, who in the previous year had discovered an inert gas, argon, in the atmosphere, identified a second inert gas (obtained from minerals containing uranium and thorium) as helium (ἥλιος, sun), an element previously revealed by spectrum analysis as a constituent of the sun. In the same year Röntgen, while experimenting with the rays that stream from the cathode in a vacuum tube, discovered new rays (which he called X-rays) possessed of wonderful photographic power. At the beginning of 1896 Henri Becquerel, experimenting on the supposition, or hypothesis, that the emission of rays was associated with phosphorescence, tested the photographic effects of a number of phosphorescent substances. He exposed, among other compounds, crystals of the double sulphate of uranium and potassium to sunlight and then placed upon the crystals a photographic plate wrapped in two thicknesses of heavy black paper. The outline of the phosphorescent substance was developed on the plate. An image of a coin was obtained by placing it between uranic salts and a photographic plate. Two or three days after reporting this result Becquerel chanced (the sunlight at the time seeming to him too intermittent for experimentation) to put away in the same drawer, and in juxtaposition, a photographic plate and these phosphorescent salts. To his surprise he obtained a clear image when the plate was developed. He now assumed the existence of invisible rays similar to X-rays. They proved capable of passing through sheets of aluminum and of copper, and of discharging electrified bodies. Days elapsed without any apparent diminution of the radiation. On the supposition that the rays might resemble light he tried to refract, reflect, and polarize them; but this hypothesis was by the experiments of Rutherford, and of Becquerel himself, ultimately overthrown. In the mean time the French scientist obtained radiations from metallic uranium and from uranous salts. These, in contrast with the uranic salts, are non-phosphorescent. Becquerel's original hypothesis was thus overthrown. Radiation is a property inherent in uranium and independent both of light and of phosphorescence.