THEORY OF THE ELECTRIC ARC 157 



or negative charge. From the derivation of the word, it 

 signifies moving, and the name is applied to nearly anything 

 that will move in an electric field and is too small to have 

 any other name. 



This definition includes ions in liquid solutions, but in 

 this discussion we shall consider only the ones which are 

 found in gases. These have different properties from those 

 of electrolytes and the phenomena of the arc are not the 

 same as those existing when a current of electricity passes 

 through a liquid. These differences can be better under- 

 stood after the facts relating to the arc have been more 

 fully discussed. 



The movement of the ions constitutes a current of elec- 

 tricity. Ions thus render the space where they exist con- 

 ducting, and by this means their presence is ordinarily 

 detected. 



The negative part which is broken off from an atom or 

 molecule is much smaller than the remaining positive part. 

 This ion which is a small fraction of an atom is often called 

 an electron. This very quickly becomes attached to an 

 atom or molecule, and though it is still an ion, it is no 

 longer called an electron. An electron, then, is a negative 

 ion which has a mass very much smaller than that of an 

 atom. In some cases of discharge through gases nearly 

 all of the negative ions are electrons. Cathode rays, for 

 example, are streams of such parts of atoms. In other 

 cases the electrons so quickly combine with molecules that 

 their presence can scarcely be detected, as when discharge 

 in air at atmospheric pressure is produced by Rontgen 

 rays. In discharge through hot flames there are probably 

 present in large numbers both electrons and negative ions 

 of atomic size. 



