A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 535 



electrostatic effects. The relation between the units employed in the two methods 

 is shewn to depend on what I have called the "electric elasticity" of the medium, 

 and to be a velocity, which has been experimentally determined by MM. Weber 

 and Kohlrausch. 



I then shew how to calculate the electrostatic capacity of a condenser, and 

 the specific inductive capacity of a dielectric. 



The case of a condenser composed of parallel layers of substances of different 

 electric resistances and inductive capacities is next examined, and it is shewn 

 that the phenomenon called electric absorption will generally occur, that is, the 

 condenser, when suddenly discharged, will after a short time shew signs of a 



residual charge. 



/ 



(20) The general equations are next applied to the case of a magnetic 

 disturbance propagated through a non-conducting field, and it is shewn that 

 the only disturbances which can be so propagated are those which are transverse 

 to the direction of propagation, and that the velocity of propagation is the 

 velocity v, found from experiments such as those of Weber, which expresses 

 the number of electrostatic units of electricity which are contained in one electro- 

 magnetic unit. 



This velocity is so nearly that of light, that it seems we have strong 

 reason to conclude that light itself (including radiant heat, and other radiations 

 if any) is an electromagnetic disturbance in the form of waves propagated through 

 the electromagnetic field according to electromagnetic laws. If so, the agree- 

 ment between the elasticity of the medium as calculated from the rapid alterna- 

 tions of luminous vibrations, and as found by the slow processes of electrical 

 experiments, shews how perfect and regular the elastic properties of the medium 

 must be when not encumbered with any matter denser than air. If the same 

 character of the elasticity is retained in dense transparent bodies, it appears 

 that the square of the index of refraction is equal to the product of the specific 

 dielectric capacity and the specific magnetic capacity. Conducting media are 

 shewn to absorb such radiations rapidly, and therefore to be generally opaque. 



The conception of the propagation of transverse magnetic disturbances to 

 the exclusion of normal ones is distinctly set forth by Professor Faraday* in 

 his "Thoughts on Kay Vibrations." The electromagnetic theory of light, as 



* Philosophical JJagazitie, May 1846, or Experimental Researches, in. p. 447. 



