Page:Popular Science Monthly Volume 87.djvu/126

122 A radioactive substance like radium emits not only alpha particles but beta particles which are electrons in very swift motion. These beta particles are generally far more penetrating than the alpha rays, but produce a much smaller number of ions per centimeter of their path

through a gas. In Fig. 10 is seen the track of a swift beta particle crossing the expansion chamber. It will be observed that the path is not straight but tortuous, due to the marked scattering of the particle by collisions with the atoms of matter in its path. Although the trail