CONTRIBUTIONS TO SCIENCE. 545 



was placed at the other end of the beam in order to elimi- 

 nate the magnetic action of the earth and the suspended 

 coil. The apparatus is now in the Cavendish Laboratory. 

 The result of the experiment gave for the ratio of the units 

 a velocity of 288,000,000 metres, or 179,000 statute miles 

 per second. The result obtained by another method by 

 MM. Weber and Kohlrausch is 310,740,000 metres per 

 second. The battery employed for the electrostatic charges 

 was M. Gassiot's battery of 2600 cells, charged with corro- 

 sive sublimate. The accuracy of this result depends on 

 that of the B. A. unit of resistance, the velocity being in fact 

 represented by 2 8 '8 Ohms. 



Now, according to the undulatory theory, light consists 

 of transverse vibrations of an elastic substance pervading 

 space and all bodies, and the velocity of light as determined 

 by Foucault is 298,000,000 metres per second, or very near 

 the mean of the values obtained by Maxwell, and by Weber 

 and Kohlrausch, for the velocity of propagation of electro- 

 magnetic disturbances. If this is found to be always the 

 case, clearly the same medium will serve to account for the 

 phenomena of electrostatics and electromagnetism, and for 

 the propagation of light which must consequently be of the 

 nature of an electromagnetic disturbance. 



If an electromagnetic disturbance take place in a perfect 

 insulator we have seen that it must be transmitted to an 

 unlimited distance, for as no slipping can take place between 

 the electric particles and the cells, and as the particles 

 themselves cannot be displaced except by inducing a cor- 

 responding elastic stress in the medium, there is no outlet 

 for the energy of the disturbance, which must therefore be 

 communicated from cell to cell without limit. But if the 

 medium be a conductor, that is, if the electric particles can 

 undergo a permanent displacement passing from molecule 

 to molecule against a frictional resistance and without any 

 tendency to return, the energy of the electromagnetic disturb- 

 ance will be gradually dissipated ; for the electric particles, 

 instead of communicating the whole of the motion of one 

 layer of cells to the next, will themselves be set in motion, 



2N 



