emission being analogous to the origin of X-rays in the phosphorescing glass walls of a vacuum tube. In accordance with this view in 1896 he exposed a photographic plate to uranium sulphate which was covered with copper and aluminium foil and found that the plate was acted upon. Accidentally he found that this action took place, no matter whether the uranium nitrate was phosphorescing or not, and he found that uranium which had never been exposed to sunlight possessed the same property. He found that these radiations from uranium were similar to the X-rays in their penetrating power. This was the first discovery of the possession of radioactivity by a body, i. e., the power of a body to ionize a gas, to affect a photographic plate or to produce phosphorescence.
Madame Curie then took up the problem of finding whether other substances possessed the properties of uranium and found that thorium did. She made a detailed investigation then of all the elements and found that none, with the exception of uranium and thorium, possessed these properties even to the order of the hundredth part of that of uranium. She, however, found that some minerals possessed a greater radioactivity than uranium or thorium, and concluded that these must contain elements more highly radioactive than either of these. After much tedious, but brilliant, work she was able to separate out the very radioactive element radium. As a result of the work of the Curies, and many others, it was found that thorium, uranium, radium and actinium were radioactive, the latter two being intensely so. None of the other elements were found to possess any radioactivity to within the limits of the experimental errors of the method of observation.
The study of these radioactive elements has been the source of very important discoveries in physics. These elements are found to emit spontaneously a continuous flight of material particles, projected with great velocity, and also to be the source of radiations similar to X-rays and called rays. We will now describe the material particles, which are of two kinds, the and rays.
The a rays consist of positively charged particles shot out by the radioactive body with a velocity approaching that of light. They are readily absorbed by thin sheets of metal foil or by a few centimeters of air. The rays are far more penetrating in character than the a rays and consist of negatively charged bodies projected with velocities of the same order of magnitude as that of light. As far as known, they are identical with the corpuscles. Of the three kinds of rays, the a rays produce the greatest amount of ionization and the rays the least. With a thin layer of unscreened radioactive matter spread on the lower of two plates, say 5 cm. apart, it will be found that the relative order of ionization due to the and rays is as 10,000 to 100 to 1, whereas the average penetrating power is inversely proportional
