[Footnote 70: _Monthly Notices_, vol. lxii., p. 619; vol. lxiii., p. 258.] [Footnote 71: _Monthly Notices_, vol. lxiii., p. 424.] [Footnote 72: J. J. Thomson, _Electricity and Matter_, p. 88.] [Footnote 73: _Ibid._, p. 47.] CHAPTER XI THE INEVITABLE ETHER Ether is the fundamental postulate of physics. Its existence, nowise apparent, is in all manner of ways implied. The properties that must be assigned to it are certainly arduous of conception. We need the aid of forced analogies to enable us to realize, even imperfectly and indistinctly, the mode in which it discharges functions obviously somehow discharged. But in the last resort everything is obscure; if our thought-borings go deep enough, they always reach the incomprehensible. The original ether was the 'quintessence' of the ancients--a kind of matter vaguely imagined as pure and incorruptible enough to serve for the raw material of the heavenly bodies, the four common elements being reserved exclusively for sublunary use. The distinction, however, eventually broke down. All the spheres, from the _primum mobile_ to the very surface of our low earth, are pervaded by a subtle mode of action, demanding for its transmission machinery of a finer kind than could be constructed out of gross everyday matter. The phenomena of light, when they came to be attentively studied, imperatively required a medium, universally diffused, evasive to sense, accessible only by processes of reasoning. Hooke and Newton accordingly brought the ether through the Horn Gate of dreamland into a region of reality, where it became a subject of legitimate speculation to men of science. The task, nevertheless, of definitely specifying its qualities was not taken seriously in hand until the beginning of the nineteenth century, when the establishment of the undulatory theory of light supplied tangible holding-ground for ideas regarding the vehicle of propagation, and rendered the ether a fixture of thought.