We have said, at page 129, that the ether does not pervade all bodies of all classes, and such must be the case in some measure at least, otherwise there would be no non-conductors of electricity, no insulators for our electric telegraphs and deep sea cables. Were glass, for instance, pervaded freely by the ether, and the ether is in reality the carrier of electricity, then electricity could pass freely through glass, but it does not; therefore, there can be no, or at all events very little, ether in glass or any other insulator. We can see, then, the possibility of the ether being removed from a glass tube, provided it is a material substance, by shutting up one end of it with a stopper of glass and passing a perfectly-fitting glass piston through it to the other end. Suppose this done, it would be quite safe to say that electricity could not pass through the tube, because there would be nothing--absolutely nothing--to carry it, not even the piston-rod, for we could have that not only made of glass but on the outside of the piston. In this case the result would be exactly the same as when the contents of the tube were pumped out of it, and the residue left, if any, would be the same, that is, an immeasurably small quantity of the ether which had filtered through the glass. It may be argued that it would be impossible to make such an experiment as we have proposed, but that does not damage in the slightest degree the correctness of the consequences deduced from it; any more than the impossibility of constructing a perfect heat engine destroys the deductions drawn by Sadi Carnot, from the study of such an ideal machine. We can grant that glass being not an absolute non-conductor, the ether might, in course of time, ooze through it and fill the tube again, while gas, air, or dust could not so ooze through it, and thus re-establish the current of electricity that was stopped for want of it; but we cannot grant that there was any very perceptible quantity of ether in the tube, when the electric current could not pass through it without dismissing the ether altogether, and dropping back into the difficulties out of which it has in many cases lifted us.