156 Royal Society. 



The relation which subsists between the electromagnetic and the 

 electrostatic system of units is then investigated, and shown to depend 

 upon what we have called the Electric Elasticity of the medium in 

 which the experiments are made (i. e. common air). Other media, 

 as glass, shellac, and sulphur have different powers as dielectrics ; 

 and some of them exhibit the phenomena of electric absorption and 

 residual discharge. 



It is then shown how a compound condenser of different materials 

 may be constructed which shall exhibit these phenomena, and it is 

 proved that the result will be the same though the different substances 

 were so intimately intermingled that the want of uniformity could not 

 be detected. 



The general equations are then applied to the foundation of the 

 Electromagnetic Theory of Light. 



Faraday, in his " Thoughts on Ilay Vibrations" *, has described the 

 effect of the sudden movement of a magnetic or electric body, and 

 the propagation of the disturbance through the field, and has stated 

 his opinion that such a disturbance must be entirely transverse to the 

 direction of propagation. In 1846 there were no data to calculate 

 the mathematical laws of such propagation, or to determine the 

 velocity. 



The equations of this paper, however, show that transverse disturb- 

 ances, and transverse disturbances only, will be propagated through 

 the field, and that the number which expresses the velocity of pro- 

 pagation must be the same as that which expresses the number of 

 electrostatic units of electricity in one electromagnetic unit, the stan- 

 dards of space and time being the same. 



The first of these results agrees, as is well known, with the undu- 

 latory theory of light as deduced from optical experiments. The 

 second may be judged of by a comparison of the electromagnetical 

 experiments of Weber and Kohlrausch with the velocity of light as 

 determined by astronomers in the heavenly spaces, and by M. Fou- 

 cault in the air of his laboratory. 



Electrostatic units inanl oir ,^ /(AAnA 

 electromagnetic unit . . . . j 310 ^ 40 > 000 metres per second. 



Velocity of light as found by M. Fizeau 314,858,000. 



Velocity of light by M. Foucault 298,000,000. 



Velocity of light deduced from aberra- ) OAO AAA AAA 

 tion ) oUy >000,000. 



At the outset of the paper, the dynamical theory of the electro- 

 magnetic field borrowed from the undulatory theory of light the use 

 of its luminiferous medium. It now restores the medium, after 

 having tested v its powers of transmitting undulations, and the cha- 

 racter of those undulations, and certifies that the vibrations are trans- 

 verse, and that the velocity is that of light. With regard to normal 

 vibrations, the electromagnetic theory does not allow of their trans- 

 mission. 



What, then, is light according to the electromagnetic theory ? It 

 consists of alternate and opposite rapidly recurring transverse mag- 

 netic disturbances, accompanied with electric displacements, the direc- 

 * Phil. Mag. 1846. Experimental Eesearches, vol. iii. p. 447. 



