BECENT DISCOVERIES IN B AVIATION. 489 



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 sub- 

 stances 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. 



Other Badiations from Badio-active Substances. 



But it was discovered in 1899 by Rutherford, of McGrill IJniversity, 

 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 infiuence 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 sub- 

 stances, which has been given the name of gamma rays. These are 

 very much more penetrating even than the beta rays; but, so far, little 



