RADIO-ACTIVITY 171 



— that IS, the extremely short electro-magnetic 

 waves produced by the cathode rays of very 

 high vacua — show absorption phenomena similar 

 to those of the 7 rays of radium. Forasmuch as 

 the P rays travel with velocities higher than those 

 of any ordinary cathode rays, we should naturally 

 expect the resulting waves to have shorter wave- 

 lengths than ordinary Rontgen rays. The nature 

 of 7 rays was finally placed beyond dispute when 

 the same experiments on crystals were carried out 

 with them as had been so successful with X-rays. 

 The 7 rays also showed diffraction phenomena, 

 and gave several spectral lines of wave-length 

 about io~^ centimetres. It seems certain then 

 that the 7 rays are identical in nature and origin 

 with very **hard" Rontgen rays. 



All the three types of radiation, when they 

 pass through air or any other gas, render the gas 

 a conductor of electricity, so that the charge of 

 an electroscope or of an electrometer leaks away. 

 The charged particles of atomic mass which 

 constitute the a rays, the negative corpuscles or 

 electrons which form the /3 rays, and the 7 rays, 

 short electric waves, are all able to convert some 

 of the molecules of a gas into electrified ions. 

 The a and (3 projectiles probably effect this 

 change by the energy of their collisions with the 

 molecules of gas, and it is possible to estimate 

 the number of ions produced by each shot. It 

 has been reckoned that this number is sufficient 

 to give air a measurable conductivity when one 

 positive particle per second is emitted by the 

 radio-active substance. Even if one atom of 

 radium emits only one such particle, this estimate 

 means that the electroscope is able to detect 



