210 SCIENCE AND METHOD. 



electrons, enjoying complete liberty, except of leaving 

 the metallic body and crossing the surface that sepa- 

 rates it from exterior space, or from the air, or from 

 any other non-metallic body. These movable elec- 

 trons behave then inside the metallic body as do the 

 molecules of a gas, according to the kinetic theory of 

 gases, inside the vessel in which the gas is contained. 

 But under the influence of a difference of potential 

 the negative movable electrons would all tend to go 

 to one side and the positive movable electrons to the 

 other. This is what produces electric currents, and it 

 is for this reason that such bodies act as conductors. 

 Moreover, the velocities of our electrons will become 

 greater as the temperature rises, if we accept the 

 analogy of the kinetic theory of gases. When one 

 of these movable electrons meets the surface of the 

 metallic body, a surface it cannot cross, it is deflected 

 like a billiard ball that has touched the cushion, and 

 its velocity undergoes a sudden change of direction. 

 But when an electron changes its direction, as we 

 shall see further on, it becomes the source of a lumin- 

 ous wave, and it is for this reason that hot metals are 

 incandescent. 



In other bodies, such as dielectric and transparent 

 bodies, the movable electrons enjoy much less liberty. 

 They remain, as it were, attached to fixed electrons 

 which attract them. The further they stray, the 

 greater becomes the attraction that tends to bring 

 them back. Accordingly the}^ can only suffer slight 

 displacements ; they cannot circulate throughout the 

 body, but only oscillate about their mean position. 

 It is for this reason that these bodies are non- 

 conductors ; they arc, moreover, generally trans- 



