172 THE ELECTRIC ARC 



to be practically uniform, which could not be, if there were 

 not an equal number of positive and negative ions through 

 the gas. 



This ionization is no doubt produced by the impact of 

 electrons on the atoms or molecules. It would seem at 

 first as if there were other possible explanations for the 

 ionization. It has been shown, for example, that ultra- 

 violet light ionizes gases. 1 Light of the short wave length 

 needed to produce such ionization is not known to exist 

 in any of the forms of the arc, but since it is completely 

 absorbed by a thin layer of air, it is quite possible that it 

 has been overlooked and does indeed exist in all forms of 

 the arc. 



Such light may indeed be an aid in producing ionization, 

 but it can not be the only source, for it comes only from 

 vibrating ions, and its energy comes from the energy given 

 to a system of ions when the molecule is broken into two 

 parts. The energy of vibration of one pair of ions will 

 not be sufficient to set another pair into similar vibration 

 of equal intensity, for a large amount of this energy is being 

 continually radiated away from the arc and another large 

 part is changed into heat and conducted away, and it would 

 require all of the energy of one vibration to produce an- 

 other like it. The number of vibrating atoms will, there- 

 fore, diminish, or some other source of energy must be 

 changed into that of vibration. In other words, if the 

 number of vibrating ions is to be kept constant the elec- 

 trical energy must in some way be changed into that of 

 ultra-violet light, and the only known way for this to be 

 done in a gas is by impact of the ions on the molecules. 



The same kind of reasoning will show that it is not 

 1 Phys. Rev., 32, i; 1911. 



