PHYSICS OF NINETEENTH CENTURY 185 



Heinrich Hertz, who in 1888 demonstrated directly 

 the passage through space of electro-magnetic waves, 

 and showed that their properties were identical with 

 those of waves of light of very great wave-length. 

 By the invention of improved methods of producing 

 and detecting these waves by Lodge, Marconi and 

 others, they have become available for practical use 

 in wireless telegraphy. 



The work of Maxwell and Hertz for a time shifted 

 the point of interest in electrical science from the 



The Theory electric charges and currents to the 

 of Ions. dielectric medium. But, in more recent 

 years, another line of investigation, simultaneously 

 pursued, has concentrated the thoughts of men of 

 science once more on the electric charge as a funda- 

 mental physical entity. 



As we have already seen, the passage of electric 

 currents through solutions of salts and acids produces 

 chemical changes at the points where the current 

 enters and leaves the liquid, the salts being there 

 decomposed chemically into their constituent parts. 

 To explain these phenomena, and to co-ordinate them 

 with the other electrical properties of solutions, it is 

 necessary to suppose that the opposite parts of the 

 salt move in opposite directions under the action of 

 the electric forces. Hence Faraday called these 

 moving particles the " ions " the travellers and 

 his conceptions have been successfully developed to 

 explain the passage of the electric current as effected 

 by the conveyance of discrete charges of electricity 

 by moving atoms or groups of atoms of the dissolved 

 salt. The motion of the ions may even be made 



