Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/476

 if the constants of the metal are such that, for a certain range of values of, is small, while  is negative, it is evident that, for this range of values of ,  will be small and  large, i.e., the properties of the metal will approach those of ideal silver. Finally, for indefinitely great values of, is small and  is nearly unity, so that  tends to unity and  to zero: an approximation to these conditions is realized in the X-rays.

In the last years of the nineteenth century, attempts were made to form more definite conceptions regarding the behaviour of electrons within metals. It will be remembered that the original theory of electrons had been proposed by Weber for the purpose of explaining the phenomena of electric currents in metallic wires. Weber, however, made but little progress towards an electric theory of metals; for being concerned chiefly with magneto-electric induction and electromagnetic ponderomotive force, be scarcely brought the metal into the discussion at all, except in the assumption that electrons of opposite signs travel with equal and opposite velocities relative to its substance. The more comprehensive scheme of his successors half a century afterwards aimed at connecting in a unified theory all the known electrical properties of metals, such as the conduction of currents according to Ohm's law, the thermo-electric effects of Seebeck, Peltier, and W. Thomson, the galvano-magnetic effect of Hall, and other phenomena which will be mentioned subsequently.

The later investigators, indeed, ranged beyond the group of purely electrical properties, and sought by aid of the theory of electrons to explain the conduction of heat. The principal ground on which this extension was justified was an experimental result obtained in 1853 by G. Wiedemann and R. Franz, who found