ether. Maxwell explained the fact that many electrolytes are transparent and yet are goou conductors by supposing that the rapidly alternating electromotive forces which occur during the transmission of the electrical disturbance act for so short a time in one direction, that no complete separation of the molecules of the electrolyte is effected. No electrical current, therefore, is set up in the electrolyte, and electrical energy is not lost during the transmission of the disturbance.
The experiments of Hertz and others, described in § 313, have proved that electromagnetic waves may be set up in a medium, and that they possess the properties predicted for them by Maxwell's theory. In very many respects these waves behave exactly like light-waves; they are transmitted with the same velocity, they move more slowly through dense bodies than iu a vacuum, they are reflected, refracted, and polarized exactly as light-waves are, and they penetrate bodies which are transparent to light, and are stopped by bodies which are opaque to light. There are certain differences between their behavior and that of light-waves, which are readily explained by the fact that the shortest electromagnetic waves which can be produced directly are several centimetres long, while none of the light-waves are as long as one one-thousandth of a millimetre. The periods of vibration of the electromagnetic waves are much greater than those of light-waves, and such properties of these waves as depend upon their periods are to some extent different from those of light.
One of the most important conclusions of the electromagnetic theory was that of the relation between the index of refraction and the specific inductive capacity. This relation is very far from being confirmed by experiment when the index of refraction is that of light. This discrepancy between theory and experiment is explained by those who maintain that light is an electromagnetic disturbance of the sort described in the following way: The methods by which the specific inductive capacity is determined either involve setting up a steady electrical force in the dielectric or the use of an alternating electrical force which at best only alternates with a
