[Footnote 9: _The Ether_. In a thoughtful lecture on the relations between light and electricity at the sixty-second Congress of German naturalists and physicians in Heidelberg in 1889, Heinrich Hertz explains the scope of his brilliant discovery: "Thus the domain of electricity extends over the whole of nature. It comes nearer to ourselves; we learn that we actually possess an electric organ, the eye. Here we are brought face to face with the question as to unmediated _actio in distans_. Is there such a thing? Not far off from this, in another direction, lies the question of the nature of electricity. And immediately connected therewith arises the momentous and primary question as to the nature of the ether, of the properties of the medium that fills all space, its structure, its rest or motion, its infinitude or finitude. It becomes every day more manifest that this question rises above all others, that a knowledge of what the ether is would reveal to us not only the nature of the old 'imponderables,' but also of the old 'matter' itself and its most essential properties, weight and inertia. Modern physics is not far from the question whether everything that exists is not created from the ether." This question is already being answered in the affirmative by some monistic physicists, as, for example, by J. G. Vogt in his most suggestive work on _The Nature of Electricity and Magnetism_, on _The Basis of the Conception of a Single Substance_ (Leipsic, 1891). He regards the atoms of mass (the primal atoms of the kinetic theory of matter) as individualised centres of concentration of the continuous substance that uninterruptedly fills all space; the mobile elastic part of this substance between the atoms, and universally distributed, is--the ether. Georg Helm in Dresden, on the basis of mathematico-physical experiments, had already at an earlier date arrived at the same conclusions; in his treatise on "Influences at a Distance mediated by the Ether" (_Annalen der Physik und Chemie_, 1881, Bd. xiv.), he shows that it requires only the postulate of one particular kind of matter, the ether, to explain influence at a distance and radiation; that is, as regards these phenomena, all the qualities ascribable to matter, except that of motion, are of no account; in other words, that in thinking of the ether we simply require to think of it as "the mobile."]