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THE POPULAR SCIENCE MONTHLY 



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 



Fig. 9. Magnified Track of Alpha Particles (Wilson). 



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 



Fig. 10. Tracks of Beta Particles. 



