A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



tion *, in virtue of which, when the dielectric is exposed to electromotive force, 

 the electric displacement gradually increases, and when the electromotive force 

 is removed, the dielectric does not instantly return to its primitive state, but 

 only discharges a portion of its electrification, and when left to itself gradually 

 acquires electrification on its surface, as the interior gradually becomes depolarized. 

 Almost all solid dielectrics exhibit this phenomenon, which gives rise to the 

 residual charge in the Leyden jar, and to several phenomena of electric cables 

 described by Mr F. Jenkint. 



(14) We have here two other kinds of yielding besides the yielding of 

 the perfect dielectric, which we have compared to a perfectly elastic body. The 

 yielding due to conductivity may be compared to that of a viscous fluid (that 

 is to say, a fluid having great internal friction), or a soft solid on which the 

 smallest force produces a permanent alteration of figure increasing with the 

 time during which the force acts. The yielding due to electric absorption may 

 be compared to that of a cellular elastic body containing a thick fluid in its 

 cavities. Such a body, when subjected to pressure, is compressed by degrees 

 on account of the gradual yielding of the thick fluid ; and when the pressure 

 is removed it does not at once recover its figure, because the elasticity of the 

 substance of the body has gradually to overcome the tenacity of the fluid before 

 it can regain complete equilibrium. 



Several solid bodies in which no such structure as we have supposed can 

 be found, seem to possess a mechanical property of this kind J ; and it seems 

 probable that the same substances, if dielectrics, may possess the analogous 

 electrical property, and if magnetic, may have corresponding properties relating 

 to the acquisition, retention, and loss of magnetic polarity. 



(15) It appears therefore that certain phenomena in electricity and mag- 

 netism lead to the same conclusion as those of optics, namely, that there is 

 an asthereal medium pervading all bodies, and modified only in degree by their 

 presence ; that the parts of this medium are capable of being set in motion 

 by electric currents and magnets ; that this motion is communicated from one 



* Faraday, Experimental Researches, 12331250. 



t Report* of British Association, 1859, p. 248; and Report of Committee of Board of Trade on 

 Submarine Cablet, pp. 136 it 464. 



J As, for instance, the composition of glue, treacle, ic., of which small plastic figures are made, 

 which after being distorted gradually recover their bhape. 



