A COMPARISON OF THE ELECTRIC UNITS, &C. 137 



Note on the Electromagnetic Theory of Light. 



In a paper on the Electromagnetic Field* some years ago, I laid before 

 the Royal Society the reasons which led me to believe that light is an elec- 

 tromagnetic phenomenon, the laws of which can be deduced from those of 

 electricity and magnetism, on the theory that all these phenomena are affections 

 of one and the same medium. Two papers appeared in Poggendorff's Anna- 

 len, for 1867, bearing on the same subject. The first, by the late eminent 

 mathematician Bernhardt Eiemann, was presented in 1858 to the Royal Society 

 of Gottingen, but was withdrawn before publication, and remained unknown 

 till last year. Riemann shews that if for Laplace's equation we substitute 



........................ (13), 



V being the electrostatic potential, and a a velocity, the results will agree 

 with known phenomena in all parts of electrical science. This equation is equi- 

 valent to a statement that the potential V is propagated through space with 

 a certain velocity. The author, however, seems to avoid making explicit 

 mention of any medium through which the propagation takes place, but he 

 shews that this velocity is nearly, if not absolutely, equal to the known velocity 

 of light. 



The second paper, by M. Lorenz, shews that, on Weber's theory, periodic 

 electric disturbances would be propagated with a velocity equal to that of light. 

 The propagation of attraction through space forms part of this hypothesis also, 

 though the medium is not explicitly recognised. 



From the assumptions of both these papers we may draw the conclusions, 

 first, that action and reaction are not always equal and opposite, and second, 

 that apparatus may be constructed to generate any amount of work from its 

 resources. 



For let two oppositely electrified bodies A and B travel along the line 

 joining them with equal velocities in the direction AB, then if either the 

 potential or the attraction of the bodies at a given time is that due to their 

 position at some former time (as these authors suppose), B, the foremost body, 

 will attract A forwards more than A attracts B backwards. 



* Philosophical Transactions, 1865, p. 459. [Vol. I. p. 527.] 

 VOL. II. 18 



