RADIO-ACTIVITY 169 



One part was found to be unable to pass through 

 more than about four layers of thin aluminium 

 foil, while the other part would pass through about 

 one hundred layers before its intensity was reduced 

 by one half. The first named, or a rays, produce 

 the most marked electric effects, while the more 

 penetrating, or /5 rays, are those which affect a 

 photographic plate through opaque screens. At 

 a later date was detected a third type of still more 

 penetrating radiation, known as 7 rays, which 

 can traverse plates of lead a centimetre thick, and 

 still produce photographs and discharge electro- 

 scopes. In proportion to its general activity, 

 radium evolves all three types of radiation much 

 more freely than uranium, and is best employed 

 for their investigation. 



The moderately penetrating or fi rays can be 

 deflected easily by a magnet ; and Becquerel, who 

 deflected them by an electric field as well, con- 

 clusively proved that they were projected particles, 

 charged with electricity. M. and Mme. Curie had 

 shown previously by direct experiment the exist- 

 ence of a negative charge associated with these 

 rays. Owing to their ionizing action, it is im- 

 possible to demonstrate that a body surrounded 

 by air gains a charge when exposed to the rays. 

 Such a charge would leak away as fast as it was 

 acquired. But, by working in a very good vacuum, 

 or by surrounding the body with a solid dielectric 

 such as paraffin, the acquisition of a negative 

 charge can be demonstrated by means of an 

 electrometer. Further investigation showed that 

 the i^ rays behave in all respects like cathode rays, 

 although they possess greater velocities than any 

 cathode rays hitherto examined, velocities which 



