by experimenting with uranium salt long kept in the dark, he found that the emission of penetrating rays, giving photographic effects, was produced spontaneously. The emission of rays by this particular sample of uranium salt has shown no sign of diminution since this discovery. The emission of penetrating rays by uranium was soon found to be independent of its phosphorescence. Phosphorescent bodies, as such, do not emit penetrating rays. Uranium compounds, whether phosphorescent or not, emit and continue to emit, these penetrating rays, capable of passing through black paper and in a less degree through metallic copper. They do not derive this property from the action of light or any other treatment. The emission of these rays discovered by Becquerel is a new property of matter. It is called ‘radio-activity,’ and the rays are called Becquerel rays. [Illustration: FIG. 9.--HENRI BECQUEREL’S DISCOVERY OF RADIO ACTIVITY. Photographic print or skiagraph of a copper Maltese Cross produced by uranium salt placed as a heap of powder on the surface of black paper wrapped round a sensitive plate. Between the paper and the uranium powder the flat copper cross was interposed. The rays from the uranium salt have penetrated the black paper, but have been intercepted to a large extent by the copper cross--so that the sensitive silver plate is darkened all about the cross--over an area corresponding to that of the heap of uranium salt, but is left pale where the copper figure blocked the path of the active rays given off by the uranium, partially but not wholly. It was thus proved that the rays from the uranium salt can pass through blackened paper and also though to a less extent through a plate of copper.]