Roentgen didn't know what the rays were, so he decided to call them X rays. He found that they could pass not only through black paper, but through many other substances as well. They went easily through cloth or wood, but were slopped by metal. Roentgen found that the mysterious rays could shine right through the soft parts of the body, but they were stopped by bones.
Wilhelm K. Roentgen
X ray of a broken leg bone
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.
