Page:Popular Science Monthly Volume 67.djvu/17

presumably a smaller velocity of projection. This result would indicate that $e/m$ is different for the a particles of polonium and radium. It is of importance to determine accurately the ratio of $e/m$ and the velocity for the rays penetrating two substances in order to settle this vital point.

 

The $\gamma$ Rays.

In addition to the $\alpha$ and $\beta$ rays, uranium, thorium and radium all emit very penetrating rays, known as $\gamma$ rays. These rays are about 100 times as penetrating as the $\beta$ rays and their presence can be detected after passing through several centimeters of lead. Villard, who originally discovered these rays in radium, stated that they were not deflected in a magnetic field, and this result has been confirmed by other observers. Quite recently, Paschen has described some experiments which led him to believe that the ty rays are corpuscular in character, consisting of negatively charged particles (electrons) projected with a velocity very nearly equal to that of light. This conclusion is based on the following evidence: Some pure radium bromide was completely enclosed in a lead envelope 1 cm. thick—a thickness sufficient to completely absorb the ordinary $\beta$ rays emitted by radium, but which allows about half of the $\gamma$ rays to escape. The lead envelope was insulated in an exhausted vessel and was found to gain a positive charge. In another experiment, the rays escaping from the lead envelope fell on an insulated metal ring, surrounding it. When the air was exhausted, this outer ring was found to gain a negative charge. These experiments, at first sight, indicate that the $\gamma$ rays carry with them a negative charge like the $\beta$ rays. In order to account for the absence of deflection of the path of the $\gamma$ rays in very strong magnetic or electric fields, it is necessary to suppose that the particles have a very large apparent mass. Paschen supposes that the $\gamma$ rays are negative electrons like the $\beta$ rays, but are projected with a velocity so nearly equal to that of light, that their apparent mass is very great.

Some experiments recently made by Mr. Eve, of McGill University, are of great interest in this connection. He found by the electric method that the $\gamma$ rays set up secondary rays, in all directions, at the surface from which they emerge and also on the surface on which they impinge. These rays are of much less penetrating power than the primary rays and are readily deflected by a magnetic field. The direction of deflection indicated that these secondary rays consisted, for the most part, of negatively charged particles (electrons) projected with sufficient velocity to penetrate through about 1 mm. of lead. In the light of these results, the experiments of Paschen receive a simple explanation without the necessity of assuming that the