hypothesis; yet [it is] no more than the observed phenomena of light and heat force us to accept. We cannot deny even the strange suggestion of Dr. Young, that there may be independent worlds, some possibly existing in different parts of space, but others perhaps pervading each other, unseen and unknown, in the same space. For if we are bound to admit the conception of this adamantine firmament, it is equally easy to admit a plurality of such." [4] Jevons's Principles of Science, Vol. II. p. 145. The figures, which in the English system of numeration read as seventeen billions, would in the American system read as seventeen trillions. The ether, therefore, is unlike any of the forms of matter which we can weigh and measure. In some respects it resembles a fluid, in some respects a solid. It is both hard and elastic to an almost inconceivable degree. It fills all material bodies like a sea in which the atoms of the material bodies are as islands, and it occupies the whole of what we call empty space. It is so sensitive that a disturbance in any part of it causes a "tremour which is felt on the surface of countless worlds." Our old experiences of matter give us no account of any substance like this; yet the undulatory theory of light obliges us to admit such a substance, and that theory is as well established as the theory of gravitation. Obviously we have here an enlargement of our experience of matter. The analysis of the phenomena of light and radiant heat has brought us into mental relations with matter in a different state from any in which we previously knew it. For the supposition that the ether may be something essentially different from matter is contradicted by all the terms we have used in describing it. Strange and contradictory as its properties may seem, are they any more strange than the properties of a gas would seem if we were for the first time to discover a gas after heretofore knowing nothing but solids and liquids? I think not; and the conclusion implied by our authors seems to me eminently probable, that in the so-called ether we have simply a state of matter more primitive than what we know as the gaseous state. Indeed, the conceptions of matter now current, and inherited from barbarous ages, are likely enough to be crude in the extreme. It is not strange that the study of such subtle agencies as heat and light should oblige us to modify them; and it will not be strange if the study of electricity should entail still further revision of our ideas.