Here was a useful discovery, indeed. Just imagine how valuable it was to a doctor to be able to take a shadow picture of a broken bone so that he could see how to set it. Your dentist probably uses Roentgen's X rays to take pictures of your teeth to look for cavities. Roentgen's discovery stirred up a great deal of interest. Perhaps there were other kinds of rays. Scientists began to search for them. One of the searchers was Antoine Henri Becquerel (Beck-er-EL), a Frenchman. In 1896, Becquerel was experimenting with some crystals of a uranium salt. These crystals gave off a glow after being exposed to sunlight. One day, Becquerel was all ready to test the glow on a photographic film. But just as he was about to start, the sky turned cloudy. The experiment couldn't be done without sunlight, so Becquerel wrapped his film in a piece of black paper, put the crystal on top, and put the package away in a drawer. [Illustration] [Illustration] The sun stayed hidden for several days and Becquerel couldn't continue with his experiment. But he decided to develop his film anyway. He was astonished to find a black spot right in the middle, just where the uranium crystal had been! That meant that even without any sunlight, the crystal had given off rays of its own! Another mystery! Becquerel tested several compounds that contained uranium and found that they all gave off similar rays. Why? [Illustration] One of the scientists who heard about Becquerel's work was a young Polish woman working in Paris. Her name was Marie Curie (Cu-REE). Madame Curie wondered if uranium were the only element to give off rays. With the help of her husband, Pierre, she began to test every known element for rays. She decided to call this ability to give off rays _radioactivity_. After many tests, Madame Curie found that the element _thorium_ was also radioactive. So, the Curies reasoned, if there were _two_ radioactive elements, there were probably more. And they continued to search. Then one day, a strange thing happened. The Curies were busy refining uranium from its ore, _pitchblende_. But they suddenly noticed that the ore seemed to be more radioactive than the uranium itself! How could this be? The only explanation, Madame Curie thought, was that there must be another, stronger, radioactive element in pitchblende. In 1898, after working through tons of pitchblende, the Curies succeeded in separating a speck of a new element that was 900 times more radioactive than uranium! They named the new element _radium_. The Curies and other scientists were very excited. Here were three elements--uranium, thorium, and radium--all giving off powerful rays. Where did the rays come from? Scientists were sure they could only come from the atoms of the elements themselves. But how could that be? There couldn't be anything smaller than an atom. Or could there? The scientists didn't know. It was a real mystery. 4 WHAT'S IN AN ATOM?