Impulsion hypotheses, whether by corpuscles or rays, being hopelessly discredited, Dr. Lorentz reverted to a half-forgotten speculation by Mosotti, which, though sixty years old, struck him as capable of being adapted to modern requirements. It was of an electrical nature, and, in the novel shape given to it, supposed gravitational action to depend upon strains of the ether due to the disturbing effects of the positive and negative ions constituting ordinary matter. These 'states' of the medium are distinct in kind; they cannot neutralize one another; and the familiar law of attraction represents their resultant effect. To bring it about much has to be taken for granted; yet the hypothesis can lay claim to one singular prerogative. Although the disturbances invoked by it traverse the ether with no more than the standard speed of light, it appears from Dr. Lorentz's investigation that, owing to certain modifications in the properties of the medium produced by moving matter, the planetary perturbations betraying loss of time in gravitational transmission would, on the electrical theory, be so small as to evade detection. As regards this crucial point, the Dutch physicist has hit upon a felicity of explanation entirely original, and, as it were, unsought. An 'undulatory theory' of gravity, adumbrated, rather than advanced by Mr. Whittaker in 1902,[70] excited hopes that the ideal aim of science--a complete unification of the forces of Nature--might at last be within reach. Based upon a striking mathematical research, it exhibited the attraction between masses as, in a manner, the integration of innumerable wave disturbances, propagated at a rate not strictly definable, but perhaps immensely surpassing that of gravity. No suggestion was made as to the primary mode or cause of agitation, yet it seemed much to learn that the medium we are cognizant of in space might be capable of transmitting the pull of gravity. Unfortunately, however, the physical foundation of this reassuring congruity proves to be weak or unsound. The mathematical mill works magnificently, but the grist put into it is of dubious quality. Stripping Mr. Whittaker's result of its purely analytical form, Dr. Johnstone Stoney showed that an assumption of extreme improbability lay concealed in his equations, which could not, he concluded, be seriously taken to correspond with the reality of things.[71] There would then seem to be no alternative but to accept _ad interim_ the electro-dynamical view of the nature of gravity. If not true, it is at least not obviously false. Through its subtlety it escapes direct confutation. And the method of exclusions, by eliminating competitors, has left it in virtual possession of the field.