Page:Elementary Text-book of Physics (Anthony, 1897).djvu/343

§ 284] is no reason to believe that such a rigorous arrangement of the ions can be made.

284. Grotthus's Theory of Electrolysis.— The foundation of all the present theories of electrolysis is found in the theory published by Grotthus in 1805. He considered the constituent ions of a molecule as oppositely electrified to an equal amount. When the current passes, owing to the electrical attractions of the electrodes, the molecules arrange themselves in lines with their similar ends in one direction, and then break up. The electro-negative ion of one molecule moves toward the positive electrode and meets the electro-positive ion of the neighboring molecule, with which it momentarily unites. At the ends of the line an electro-negative ion with its charge is freed at the anode, and an electro-positive ion with its charge is freed at the cathode. This process is repeated indefinitely so long as the current passes.

Faraday modified this view, in that he ascribed the arrangement of the molecules, and their disruption, to the stress in the medium which was the cardinal point in his electrical theories. Otherwise he held closely to Grotthus's theory. He showed that an electrical stress exists in the electrolyte by means of fine silk threads immersed in it. These arranged themselves along the lines of electrical stress.

Other phenomena, however, show that Grotthus's hypothesis can only be treated as a rough illustration of the main facts. Joule showed that during electrolysis there is a development of heat at the electrodes, in certain cases, which is not accounted for by the elementary theory above given. It must depend upon a more complicated process of electrolysis than the one we have described.

The results of researches on the so-called migration of the ions are also at variance with Grotthus's theory. If the electrolysis of a copper salt, in a cell with a copper anode at the bottom, be examined, it will be found that the solution becomes more concentrated about the anode and more dilute about the cathode. These changes can be detected by the color of the parts of the solution, and substantiated by chemical analysis. If this result be explained