was to expose uranium to strong sunlight for a long time, and then to notice whether a photographic plate, which was wrapped up carefully in perfectly opaque paper and placed beneath the uranium, received any impression from it. He found that it did; but he further found that the exposure of the uranium to sunlight was altogether unnecessary; that the uranium itself in a perfectly dark room would affect, in the course of ten or twenty days, a photographic plate from which it was separated both by opaque black paper and by a thin sheet of metal. In fact he obtained in this way a radiograph of a metallic object similar in all respects to the pictures which Röntgen had obtained with X-rays. This showed, in the first place, that the fluorescent light had nothing whatever to do with the production of the photograph, but it showed also something much more important than this, namely, that the mineral uranium is all the time spontaneously emitting rays of some sort, which are capable of penetrating opaque objects in just the way the X-rays do.
This discovery, which has been one of the most fruitful in the history of science, is immediately due to the accident of a few cloudy days in Paris, during which Becquerel, since he could not expose his uranium to sunlight, set away his plate with the uranium on the top of it, to wait for fair weather. When the fair weather returned and he was ready to continue his experiments, it fortunately occurred to him that it might be worth while to develop the plate upon which the uranium had rested to see if anything had happened to it. The discovery of radio-activity was the result. Those who recall the story of the discovery of photography will remember that it was made quite as accidentally and under quite similar circumstances.
Becquerel further found that the rays emitted by uranium are also emitted by all uranium compounds. He therefore named them uranium rays. Another property which he found that the rays possessed, in addition to that of affecting a photographic plate, was the important property of rendering a gas through which they pass a conductor of electricity, or, to state the same thing in another way, the property of discharging any electrified body which is brought into their neighborhood.
The Discovery of Radium.
It was but a few months after this that Madame Curie, one of the few women who has attained eminence in the pursuit of science, and who together with her husband, with whom most of her work has been done, deserves a large share of the credit for our present knowledge of radium, set about investigating all the then known elements to see if any of the rest of them possessed this remarkable property which Becquerel discovered in uranium. She found that one, and but one, of the remainder of the elements, namely, thorium, the element which is one of the chief constituents of Welsbach mantles, was capable of pro-
