have no grounds for choosing. But it would seem that the third, whether strictly true or not, best represents the state of the case relatively to our feeble capacity of comprehension. Whether absolutely infinite or not, the dimensions of the universe must be taken as practically infinite, so far as human thought is concerned. They immeasurably transcend the capabilities of any gauge we can bring to bear on them. Accordingly all that we are really entitled to hold, as the outcome of sound speculation, is the conception of innumerable systems of worlds concentrating out of nebulous masses, and then rushing together and dissolving into similar masses, as bubbles unite and break up--now here, now there--in their play on the surface of a pool, and to this tremendous series of events we can assign neither a beginning nor an end. [3] Fortnightly Review, April, 1875. We must now make some more explicit mention of the ether which carries through space the rays of heat and light. In closest connection with the visible stellar universe, the vicissitudes of which we have briefly traced, the all-pervading ether constitutes a sort of unseen world remarkable enough from any point of view, but to which the theory of our authors ascribes capacities hitherto unsuspected by science. The very existence of an ocean of ether enveloping the molecules of material bodies has been doubted or denied by many eminent physicists, though of course none have called in question the necessity for some interstellar medium for the transmission of thermal and luminous vibrations. This scepticism has been, I think, partially justified by the many difficulties encompassing the conception, into which, however, we need not here enter. That light and heat cannot be conveyed by any of the ordinary sensible forms of matter is unquestionable. None of the forms of sensible matter can be imagined sufficiently elastic to propagate wave-motion at the rate of one hundred and eighty-eight thousand miles per second. Yet a ray of light is a series of waves, and implies some substance in which the waves occur. The substance required is one which seems to possess strangely contradictory properties. It is commonly regarded as an "ether" or infinitely rare substance; but, as Professor Jevons observes, we might as well regard it as an infinitely solid "adamant." "Sir John Herschel has calculated the amount of force which may be supposed, according to the undulatory theory of light, to be exerted at each point in space, and finds it to be 1,148,000,000,000 times the elastic force of ordinary air at the earth's surface, so that the pressure of the ether upon a square inch of surface must be about 17,000,000,000,000, or seventeen billions of pounds." [4] Yet at the same time the resistance offered by the ether to the planetary motions is too minute to be appreciable. "All our ordinary notions," says Professor Jevons, "must be laid aside in contemplating such an