It is a strange thought that the objects constituting our universe, so long regarded by man as the only universe, are in a sense pervaded by the materials of an utterly different universe,--which yet is as essential to our very existence as what we commonly call matter. We cannot live without light and heat, for instance, and again, light and heat affect matter as we know it; but they thus exist and affect such matter by means only of a form of matter unlike any which we can conceive. It is certain that if absolute vacancy separated our earth from the sun, even by the narrowest imaginable gap, his heat and light could never reach us. They could no more pass that vacant space than the wave-motion of water can cross a space where water itself is wanting. It is because of relations such as these that it has been said, and justly, that matter is the less important half of the material constituting the physical universe. Our knowledge of this universe within our universe has been obtained within comparatively recent years. Men were unwilling or at least they spoke and thought as if they were unwilling, to believe that the universe of matter which they had so long recognised was dependent on another universe for its chief if not all its properties. They regarded heat as some sort of substance, which might, with more delicate means than they possessed, admit of being dealt with as chemists had dealt with the gases. The sun was full of this fluid, this phlogiston, as it was called. Light, in so far as it could be distinguished from heat, was another fluid; electricity was another. These were the imponderables, or unweighable substances of last century's science,--not as with us, the effects of modes of motion taking place in a universe which, though material, is yet not made of matter such as we know, or even such as we can at present conceive. This is the greatest of all human scientific marvels,--the greatest because it includes all others. We know of a universe which is as infinite in extent, and doubtless in duration, as our own universe; which pervades all forms of matter: and yet we know of this universe only indirectly; by the effects of movements taking place within it, not by any perception of these movements themselves. Waves are ever beating upon the shores of our material universe, and constantly changing the form and condition of the coast line, but the waves themselves are unseen. We only know of their existence through the changes wrought by them.