222 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



physical action which enables a voltaic cell or dynamo to exert 

 the required electromotive force are not matters for consideration, 

 although, as a matter of fact, much more is known concerning 

 the nature of electric field than is known concerning the nature 

 of mechanical stresses in substances like steel. 



The science of mechanics, as stated above, deals with those 

 phenomena which depend upon the mutual actions of bodies in 

 bulk. The phenomena of chemical action and those physical 

 phenomena which have to do with the minute details of physical 

 processes, however, have been studied heretofore almost solely 

 on the basis of the atomic theory. Thus, nearly the whole of 

 chemistry is based on the atomic theory ; the kinetic theory of 

 gases is a branch of the atomic theory ; the theory of crystal for- 

 mation is a branch of the atomic theory ; the study of the phe- 

 nomena of electrolysis is a branch of the atomic theory ; and the 

 study of the phenomena of the discharge of electricity through 

 gases is a branch of the atomic theory. 



126. Electrons and ions. The loss of electricity from a 

 charged bqdy has long been known to be due in part to a leak- 

 age of 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. The electrical conductivity of the air is ordinarily 

 extremely small, but there are a number of influences which 

 cause the air (or any gas) to become a fairly good electrical con- 

 ductor. Thus, a gas becomes a fairly good conductor when its 

 temperature is raised above a certain point ; gas which is drawn 

 from the neighborhood of a flame or electric arc, 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 conduc- 

 tor ; and the passage, through a gas, of ultra-violet light, of 

 Roentgen rays, or of the radiations from radio-active substances, 

 causes the gas to become a fairly good conductor. The conduc- 



