324 CLASSICS OF MODERN SCIENCE 

 the nonconducting plate of the condenser, and that precisely what 

 brings it to cessation is the opposite electromotive force set up by the 

 displacement of electricity in this dielectric. 



Currents become sensible in three ways — by their heating effects, 

 by their actions on other currents and on magnets, and by the induced 

 currents to which they give rise. We have seen why currents of con- 

 duction develop heat and why currents of displacement do not. But 

 Maxwell's hypothetical currents ought at any rate to produce electro- 

 magnetic and inductive effects. Why do these effects not appear? 

 The answer is, that it is because a current of displacement can not last 

 long enough. That is to say, they can not last long in one direc- 

 tion. Consequently in a dielectric no current can long exist with- 

 out alteration. But the effects ought to and will become observ- 

 able if the current is continually reversed at sufficiently short inter- 

 vals. 



THE NATURE OF LIGHT 



Such, according to Maxwell, is the origin of light. A luminiferous 

 wave is a series of alternating currents produced in dielectrics, in air, 

 or even in the interplanetary void, and reversed in direction a million 

 of million of times per second. The enormous induction due to these 

 frequent alternations sets up other currents in the neighboring parts 

 of the dielectric, and so the waves are propagated. 



Calculation shows that the velocity of propagation would be equal 

 to the ratio of the units, which we know is the velocity of light. 



Those alternative currents are a sort of electrical oscillation. Are 

 they longitudinal, like those of sound, or are they transversal, like 

 those of Fresnal's ether? In the case of sound the air undergoes al- 

 ternative condensations and rarefactions. The ether of Fresnal, on 

 the other hand, behaves as if it were composed of incompressible 

 layers capable only of slipping over one another. Were these cur- 

 rents in open paths, the electricity carried from one end to the 

 other would become accumulated at one extremity. It would 

 thus be condensed and rarefied like air, and its vibrations would 

 be longitudinal. But Maxwell only admits currents in closed 

 circuits; accumulation is impossible, and electricity behaves like 



