118 CHEMICAL DISCOVERY AND INVENTION 



The cathode stream was found to consist not of atoms or 

 molecules, but something much smaller which were at first 

 spoken of merely as corpuscles, but the convenient term electron, 

 introduced by Johnstone Stoney, was soon adopted, and has been 

 generally used ever since. The word electron was originally 

 employed to designate the atom of electricity or electrolytic 

 unit which is carried by an atom of hydrogen in electrolysis (q.v.). 



The mass of each of the cathode particles is about T sW of the 

 mass of an atom of hydrogen, the smallest of known atoms. 

 But the properties of the electron seem to have no connection 

 with the nature of the gas through which the discharge takes 

 place, nor with the material of the cathode itself. It is now 

 known that similar electrons are emitted not only from ordinary 

 atmospheric gases, but from the inert elements helium, argon, 

 and the rest. Solids of many kinds, red-hot metals or metal 

 surfaces illuminated by ultraviolet rays, as well as such oxides 

 as lime and baryta, when heated also yield electrons. Radium 

 and other radio-active substances emit them, and when derived 

 from this source they are spoken of as /2-rays. In fact electrons are 

 to be found in all directions, and probably play an important part 

 in many natural phenomena previously obscure. Thus it is sur- 

 mised that the aurora seen in the northern sky is produced by 

 electrons discharged from the sun and moving under the influence 

 of the earth's magnetic lines of force. 



There remains one important property of the electron to 

 which no reference has so far been made, and that is its power 

 of ionising gases. Under ordinary conditions air and other dry 

 gas is an almost perfect non-conductor of electricity. But the 

 residual gas in a vacuum tube through which a discharge is 

 passing acquires conductivity, and at the same time becomes 

 more or less luminous. Air in the presence of radium, as will be 

 explained later, also acquires the power of conducting electricity. 

 The ions thus produced are some of them positive and some 

 negative ; they are thought to be formed by the removal of a 

 negative electron from a molecule of the gas which thus becomes 

 positive, while the negative,,electron may attach itself to another 

 molecule of the gas forming the negative ion. It seems also 

 possible that they may be formed of fragments of molecules 

 which have been broken up by collisions with the swiftly moving 

 electrons. The latter gradually lose their energy in the process 

 and ultimately lose their cathodic character, possibly entering 



