The X-rays were not the only kind of new rays discovered towards the close of the nineteenth century. In fact they ushered in the discovery of quite a number of new kinds of radiation, each more wonderful than the last. We have already mentioned the green glow on the glass, and the white light on the platinum screen which catches the electrons. Somebody put forward the supposition that it is the phosphorescent light itself which sends out the X-rays. He supposed that whenever anything shines with phosphorescent light, X-rays are sent out, and these make an impression upon a photographic plate. This supposition was tested by a great Frenchman of the name of Becquerel. His father had made a great name by investigating phosphorescence, and what more natural than that the son should study the behaviour of phosphorescent bodies as regards X-rays! But by a very extraordinary and lucky chance he chose among the many phosphorescent bodies known to him the only body that could have led him to the great new discovery which he made. That body was a substance called uranium. He exposed some crystals containing uranium to sunlight until they glowed with a thin blue light. [Illustration: Fig. 75.--Henri Becquerel.] Then he shut up those crystals in a dark box with some photographic paper, and left them there for a few hours. At the end of that time he found that the crystals had blackened the photographic paper. This, he thought, was either the effect of the bluish light, or the effect of X-rays from the crystals. He decided to try the experiment again, but he waited in vain day after day for the sun to come out. This, as the sequel showed, was another fortunate chance. For all this time the crystals lay ready in a drawer in the neighbourhood of a photographic plate, waiting to be exposed to the sun. Tired of waiting any longer, Becquerel thought he had better close the experiment for the present by developing the photographic plate. To his great surprise, he found that the crystals had made a strong impression upon the plate, although it had been protected by black paper, and although the crystals had not been exposed to sunlight. It then struck him that perhaps the crystals had been exposed to the sun some considerable time before, and were now only very gradually giving up the light which they had absorbed. He therefore kept them for months in complete darkness, and then exposed another plate to their action. But contrary to all expectation, he got the same result as before, so that it was clear that the action of the uranium was quite independent of light, and continued without interruption.