252 CONNECTION OF ELECTEICITY AND MAGNETISM. 



Another way has been entered upon by Faraday. He objected to all 

 forces acting into distance without intermediate links of connection, 

 and he endeavored to show that electric forces spread out by contigu- 

 ous modifications of the medinm, filling the space between the electri- 

 fied bodies. He succeeded, at least, in proving that electric as well as 

 magnetic action into distance is not independent of the medium through 

 which it is propagated; and the phenomena of diamagnetism, discov- 

 ered by him, go on in such a way as if diamagnetic bodies had even 

 less magnetic polarity than the ether of a vacuum. Faraday concluded 

 from his researches that the molecules of electric insulators (dielectric 

 media) are influenced in the same way by electric forces as the molecules 

 of magnetizable bodies are influenced by magnetism; that a separation 

 of the two electricities, or, as he calls it, dielectric polarisation, goes on, in 

 the first case, as there is a separation of the magnetic fluids, or magnetic 

 polariisatio7i in the second. Prof. C. Maxwell has brought these concep- 

 tions of Faraday into a precise mathematical form. He dispenses com- 

 pletely with forces acting at a distance, and assumes actions going on in 

 every elementary volume of ether of the same direction and of the same 

 kind as the electro-magnetic actions between magnets and galvanic con- 

 ductors of common size and distance. According to his opinion, every 

 change of magnetic polarization in an elementary volume of ether pro- 

 duces in the same element an electric force of a circular direction around 

 that of the magnetic momentum, and every change of dielectric polariza- 

 tion, which is equivalent to a flow of electricity through the molecule, 

 produces magnetic force, acting in a circular direction through the same 

 molecule. In the interior of electric conductors he supposes that dielec- 

 tric polarization is continually fading away by a certain imperfect elas- 

 ticity of the ether. These assumptions give, indeed, a sufficient basis 

 for the development of a complete mathematical theory of electro-static, 

 electro-dynamic, and magnetic phenomena. It is in perfect accordance 

 with the results of experiments hitherto performed, and with the laws 

 derived from the existence of an electro-dynamic potential, at least for 

 moderate distances, through which light is propagated in a time the 

 duration of which may be neglected. But the theory of Maxwell also 

 indicates that electro-dynamic action is not proi)agated instantaneously 

 into distance, and it is highly remarkable that the velocity of the propa- 

 gation, calculated from experimental data which were obtained at first 

 by W. Weber, coincides almost perfectly with the velocity of light. 

 This coincidence had been remarked before by Kirchotf for the propa- 

 gation of electric currents in conductors of infinitely small resistance. 

 An ether, indeed, with the faculty of electric and magnetic polarization, 

 which Professor Maxwell ascribes to the ether of insulating media, can 

 propagate transverse electric and magnetic oscillations with the veloc- 

 ity of light. Magnetic and electric oscillations must be combined in 

 this case always in such a way that their directions are perpendicular 

 to each other and to the direction of propagation. The old uudulatory 



