The Groundicork of the Unwerse, &c. 47 



II.— THE RISE OF KADIO-ACTIVITY. 



The l:)cgiiiiiings of tlie science of nidio-;ictivity must be dated 

 from 187D, when Sir William Crookes published the results of 

 his investigations upon the passage of an electric discharge through 

 highly evacuated tubes. Previous to this time it was known that 

 a spark passed between two electrodes at atmospheric pressure 

 became broadened out into a columnar form as the pressure became 

 reduced, so that when a discharge was passed between the 

 electrodes of a tube containing a gas at a moderately low pressure, 

 the whole tube glowed brilliantly. 



Crookes showed that when the evacuation is carried to a 

 vei-y high degree and only a very slight amount of i-esidual gas 

 remains in the tube, the glow vanishes and the interior of the 

 tube becomes almost non-luminous. At the same time a green 

 fluoresceut light makes its appearance on the glass of the tube 

 opposite the cathode. 



Certain "rays" originate at the cathode, leave its surface 

 at right angles and travel in straight lines through the tube until 

 they impinge upon the glass, the green fluorescence occurring at 

 the points of impact. The following evidence determines the 

 character of these rays. They leave the cathode only on the 

 side nearest the positive electrode, and do not spread in all 

 directions like the radiations from a heated body. If an aluminium 

 cross be interposed in the path of the rays " a shadow of the 

 cross is thrown on the patch of green fluorescence, showing that 

 the "rays" are either vibrations like light or are streams of 

 particles. If a tiny windmill be placed in the path of the "rays" 

 it spins with great velocity, in a manner not to be explained by 

 "light pressure," but easily understood if there be a stream of 

 particles. But the "rays" can be deviated from their straight 

 path by means of a magnet or an electric field, which indicates 

 that they are jets of particles each carrying an electric charge. 

 These charges are negative when examined by an electroscope. 

 Finally, investigation shows that these parti6les have, almost 



