to interstellar or free ether. When I speak of the ether anywhere as "free," I mean that its properties are identical with the interstellar ether enormously distant from all gross matter. And this is the condition of ordinary space, except for the presence of meteoric particles, whose influence, if any, we at present legitimately ignore.
The only hypothesis which at first sight appears to assume infinitely distant ether to be affected by the motion of, say, the earth, is that of Sir George Stokes, in 1845, where an irrotational motion, zero with respect to the earth, was postulated for it. But he must have seen some way in which so impossible an assumption could be avoided; and the question how far any kind of irrotational motion can be conceived of as allowing rest at infinity, and yet no slip at the earth's surface, will be discussed later, § 31.
(ii.) Inside material bodies the ether is modified.—We learn this by direct experiment and observation.
For transparent bodies we learn it by optical experiments, which proves that light travels more slowly through their modified ether than it does in free ether; while at the same time there is no doubt but that the ether inter-penetrates them, because material substance itself is wholly incompetent to transmit anything possessing the properties and the speed of radiation.
In metallic bodies we find great opacity combined with anomalous dispersion and other complex effects, In them evidently the ether is intensely modified, if it exists at all.
I shall call the ether inside gross matter of any kind "modified ether," but as to the particular way it is modified I make no assumption. [Electrostatic experiments suggest that inside transparent bodies, something which may be called its "virtual elasticity" is diminished. Magnetic experiments suggest that inside several opaque substances it is loaded, so as to increase what may be called its "virtual density;" and there is a temptation to identify with the one, and with the other, of these two ethereal constants. Further, electrokinetic experiments suggest that inside metallic conductors the ether has a virtual viscosity, whereby its motion through matter is resisted precisely as the first power of the velocity. But none of these doubtful hypotheses shall here be obtruded.]
Rate of Travel of "Modified" Ether.
3. Defined in this way it is quite obvious that "modified ether " travels at the same steady pace as its material encasement. For lift a lump of glass or of copper from one side of a table to the other, the modified ether which was in one place is now in another, and has necessarily accompanied the material body. If the modification of ether by matter requires time, there would be some lag during epochs of acceleration; but during steady velocity there would even so be no difference in speed between modified ether and matter, only a slight lag in space.
