THE PROGRESS OF PHYSICS. 161 



American Joseph Henry (1799-1878), who also 

 detected the influence of magnetism upon electricity 

 and the phenomenon of induction-currents. 



Another of Faraday's achievements has already 

 been referred to in the chapter on chemistry, the 

 discovery of the laws of electrolysis. He showed 

 that the amount of water decomposed or gas set free 

 is strictly proportional to the quantity of electricity 

 passing through, and that equal quantities of elec- 

 tricity decompose equivalent amounts of different 

 electrolytes. 



In the third place Faraday thought out a dy- 

 namical theory of electricity, which replaced the old 

 two-fluid theory, and has formed the foundation on 

 which Kelvin, llaxwell, Helmholtz, and others have 

 reared an elaborate superstructure. While Coulomb 

 and others had assumed the possibility of " action at 

 a distance," and supposed that electric charges may 

 influence one another without any intervening me- 

 dium, Faraday's ideas were distinctly opposed to this 

 view, for he supposed that electric attraction and re- 

 pulsion were propagated by molecular agitations in 

 the particles of the insulating media which he termed 

 " dielectrics." He found reason to believe that in- 

 ductive influence takes effect along curved lines 

 (" lines of force ") and by the action of adjacent par- 

 ticles in the insulating medium. As the intensity 

 of the electric influence between two charged bodies 

 varies with the nature of the " dielectric," he was led, 

 as Cavendish had been, to the recognition of " specific 

 inductive capacity " a factor of fundamental im- 

 portance. As Cajori points out, Faraday's theory 

 gave a death-blow both to the old fluid theory and to 

 the assumption of action at a distance. 



Uaxwdl. What Faraday had expressed in his 



