devised and successfully carried out[1] for determining experimentally the kinetic energy possessed by the ions after emission. The mean kinetic energy of both negative and positive ions was found to be the same for various metals (platinum, gold, silver, etc.), and to be directly proportional to the absolute temperature, and the distribution of velocities among the ions proved to be that expressed by Maxwell's law. The ions may therefore be regarded as kinetically equivalent to the molecules of & gas whose temperature is the same as that of the metal.
By the investigations which have been recorded, the hypothesis of atomic electric charges has been, to all appearances, decisively established. But all the parts of the theory of electrons do not enjoy an equal degree of security; and in particular, it is possible that the future may bring important changes in the conception of the aether. The hope was formerly entertained of discovering an aether by reference to which motion might be estimated absolutely; but such a hope has been destroyed by the researches which have sprung from FitzGerald's hypothesis of contraction, and in some recent writings it is possible to recognize a tendency to replace the classical aether by other conceptions, which, however, have been as yet but indistinctly outlined.
In any event, the close of the nineteenth century brought to an end a well-marked era in the history of natural philosophy: and this is true not only with respect to the discoveries themselves, but also in regard to the conditions of scientific organization and endeavour, which in the last decades of that period became profoundly changed. The investigators who advanced the theories of aether and electricity, from the time of Descartes to that of Lord Kelvin, were, with very few exceptions, congregated within a narrow territory: from Dublin to the western provinces of Russia, and from Stockholm to the north of Italy, may be circumscribed by a circle of no more than six
- ↑ O. W. Richardson and F. C. Brown, Phil. Mag. xvi (1908), pp. 353, 890; F. O. Brown, Phil. Mag. xvii (1909), p. 375; xviii (1909), p. 649
