44 THE FOUNDATIONS OF SCIENCE 



a series of electro-magnetic waves, and this theory 

 was immensely strengthened by Hertz (1859-1894), 

 who, twenty years after Maxwell, succeeded in pro- 

 ducing long-wave electro-magnetic disturbances by 

 purely electrical means, and in demonstrating their 

 existence and properties. Improvements in the de- 

 tails of the apparatus soon made possible the use of 

 wireless telegraphy. 



The chemical effects of electrical currents were 

 the first of their properties to be examined, and in 

 after years led to the most striking of the triumphs 

 of modern physics in the results of the study of the 

 conduction of electricity through gases. 



When an electric current is passed between metallic 

 terminals or electrodes through the solution of a 

 salt, the salt is decomposed chemically. The products 

 of the decomposition appear at the electrodes only ; 

 the body of the solution is unaltered. To explain these 

 facts, it is necessary to suppose that a movement 

 in opposite directions of the opposite parts of the 

 salt is going on. From the conductivity of the 

 solution it is possible to calculate the relative velocity 

 with which these moving parts, or ions travellers 

 make their way through the liquid. The calculated 

 velocities have been confirmed experimentally by 

 watching the motion of coloured ions. 



A relation discovered by Arrhenius, a Swedish 

 physicist, between the conductivities and the chemical 

 activities of solutions, made it plain that, in some 



