RELATIONS TO OTHER SCIENCES 123 



interesting phenomenon observed recently by P. Villard in the 

 effect of an intense magnetic field on the production of the cathode 

 rays, appears to receive a simple explanation in the hypothesis of 

 free magnetic charges; but it is not certain that this hypothesis is 

 necessary. 



(2) The Equations of Hertz. The two fields, electric and magnetic, 

 of which the ether can be the seat, are related to one another in such 

 a manner that one of them can exist only on the condition that the 

 other varies; all variations of an electric field produce a magnetic 

 field; it is the displacement current of Maxwell: and all variations 

 of the magnetic field produce an electric field ; this is the phenome- 

 non of induction discovered by Faraday. These two relations are 

 expressed by Hertz's equations; they sum up completely our know- 

 ledge of the electromagnetic medium, and from these it results that 

 all disturbances of this medium are propagated with the velocity 

 of light. Hertz had the glory of proving this fact experimentally. 



(3) Energy. We can now say that the ether is the seat of two 

 distinct forms of energy, the electric and the magnetic, capable of 

 transformation from the one into the other, but only through matter 

 as an intermediary, that is to say, by means of the electrified centres 

 which it contains. 



In the ether alone, in fact, in the free radiation which it propagates, 

 the electric and magnetic fields, transverse with respect to the direc- 

 tion of propagation, represent always equal energies in each element 

 of volume, without oscillation of the energy from one form to the 

 other. In the presence of matter, on the other hand, the electric 

 energy can exist alone, and it is the motion of electrified centres 

 which allows the transformation into magnetic energy, and vice 

 versa. Matter only can be the source of radiation. 



It is necessary, to the two preceding forms of energy, to add 

 gravitation, which corresponds probably to a third mode of activity 

 of the ether, whose connection with the two others is still obscure. 



I insist here on the point that the principle of equivalence of vari- 

 ous forms of energy, as far as the process allows of measurement, 

 can be attained independently of all dynamical notions, by the 

 process of using solely material systems in equilibrium. 



One can find some information on this subject in a recent exposi- 

 tion by M. Perrin. 1 



(4) The Theory of Lorentz. The ether being thus completely 

 known to us from the electromagnetic and optical point of view, the 

 problem which follows as a continuation of the work of Maxwell and 

 of Hertz is that of the connection between ether and matter, inert 

 matter, the source and recipient of the radiations which the ether 



1 I. Perrin, Traits de chimie Physique. Les Principes. Gauthier-Villars, Paris. 



