Page:Popular Science Monthly Volume 64.djvu/493

Rh their course by electric charges just like the cathode rays, and, lastly, that, also like them, they imparted negative charges of electricity to objects upon which they fell. Further, when the mass of these particles was calculated by comparing the amount of deflection produced by a magnet with that produced by an electric charge, it proved to be, strangely enough, the same as that of the cathode ray particles. It seems certain, therefore, that radio-active substances spontaneously emit rays which are identical in all respects with the cathode rays, i. e., which consist of minute negatively charged particles of about one one-thousandth the size of the hydrogen atom. The velocity with which these minute particles are shot off from the radio-active substances is found to be even more enormous than the velocity of the same particles in the cathode rays. The latter were found to move with a velocity which is sometimes as high as 20,000 miles per second. Now, the velocity with which light travels from the sun to the earth or from star to star is 186,000 miles per second. Hence, the cathode ray particles sometimes move with a tenth the velocity of light. But the velocity of the particles shot off from radio-active substances is still more surprising, for it sometimes reaches the stupendous figure of 175,000 miles per second, only a trifle less than that of light.

But it was discovered in 1899 by Rutherford, of McGill University, Canada, that uranium, thorium and radium all emit other rays besides cathode rays, which are distinguishable from them, first by their very much smaller penetrating power and, second, by the fact that they are not ordinarily deviated either by a magnet or by an electrically charged body. He named these rays the alpha rays, while he designated the cathode rays emitted by radio-active substances as the beta rays. In order to separate the alpha from the beta rays, it was only necessary to lay over the radio-active substance, that is, the uranium, the thorium or the radium, a very thin sheet of aluminum; for example, a sheet .005 centimeter thick. This opposed almost no obstruction to the passage of the beta rays, but it cut off entirely the alpha rays. Another mark of difference between the two kinds of rays was that, while the beta rays were very much more effective than the alpha rays in penetrating opaque objects and in affecting a photographic plate, their influence in rendering a gas electrically conducting was very small in comparison with that of the alpha rays; so that if the thin sheet of aluminum were taken away, the gas above the radio-active substance became a hundred times as good a conductor as when the alpha rays were screened off.

There is also a third kind of ray given off by radio-active substances, which has been given the name of gamma rays. These are very much more penetrating even than the beta rays; but, so far, little