THE ELECTRON THEORY. 277 



the electricity through the surrounding air and in part to a 

 leakage of the electricity through the insulating supports of the 

 charged body, that is to say the air conducts electricity to some 

 extent. Ordinarily, however, the air is an extremely poor con- 

 ductor, but there are a number of influences which cause the air 

 (or any gas) to become a fairly good electrical conductor. Thus 

 gas which is drawn from the neighborhood of a flame or from the 

 neighborhood of glowing metal or carbon is a fairly good con- 

 ductor; gas which has been drawn from a region through which 

 an electric discharge has recently passed is a fairly good con- 

 ductor; a gas becomes a fairly good conductor under the action 

 of Roentgen rays or under the action of the radiation from radio- 

 active substances. The conductivity which is imparted to a gas 

 by these various agencies may be destroyed by filtering the gas 

 through glass wool or by placing the gas for a few moments 

 between electrically charged metal plates. This effect of filtra- 

 tion seems to show that the conductivity of the gas is due to 

 something which is mixed with the gas, and the effect of the 

 electric field (between the two charged plates) seems to show that 

 this something is charged with electricity and is dragged out of the 

 gas by the electric field. From some such considerations as these 

 the hypothesis was originated that the electrical conductivity of a 

 gas is due to electrically charged particles floating around in the 

 gas. These particles are called ions, and the process by which a 

 gas is made into a conductor is called ionization. This hypothesis 

 has been used extensively and with remarkable success in the 

 study of electrical discharge through gases and in the study of 

 radio-activity, and it is no longer thought of as a mere hypothesis. 

 The electron is a negatively charged particle of which the mass 

 is about 1/800 of the mass of a hydrogen atom. The cathode 

 rays, which are described later, are electrons thrown off from the 

 cathode of the Crooks' tube at high velocity, the 0-rays from a 

 radio-active substance such as uranium are electrons which are 

 expelled at extremely high velocity from the atoms of the sub- 

 stance. 



