EXPLORING THE ATOM 



The electron was first revealed in the cathode ray 

 which, as we have seen, is generated in a vacuum 

 tube when electricity passes through it. The cathode 

 ray, as such, consists of a stream of electrons driven 

 off from the negative pole or cathode. The fact that 

 these particles are deflected by an electric current 

 shows that they are tangible substances, and the 

 amount of deflection with a given current makes it 

 possible to compute the charge of electricity they 

 carry. 



The electron appears again as the constituent of 

 the so-called beta ray given off by a radioactive sub- 

 stance. Electrons may also be liberated from ordi- 

 nary matter, when any substance is heated to a very 

 high degree, or when rays of ultra-violet light im- 

 pinge on a metal at ordinary temperature. .Yet again 

 they appear in ordinary gases when a heavy charge 

 of electricity is forced through the gas, say a light- 

 ning stroke. They are likewise liberated in a gas 

 subjected to the X-ray or to the so-called gamma ray, 

 of radium. 



It required a vast deal of experimenting to show 

 that the electrified particles which appear under 

 these diverse circumstances are in reality one and 

 the same thing. The demonstration was made, how- 

 ever, and several means were found of testing the 

 quantity of electricity which the particle carries and 

 even of counting the particles themselves. 



The simplest method of counting the particles is 

 by passing the X-ray through a portion of air, and 

 then allowing the air to expand in a receptacle. Ex- 

 pansion cools the air and causes the deposit of drop- 



127 



