III.
OF THE INFINITELY MINUTE.
When I speak of the infinitely minute, I use the word infinitely not in its absolute sense, but relatively. Actual infinity of minuteness is as utterly beyond our conceptions as actual infinity of vastness. But we may speak of what is very much less than the least object of which our senses can make us directly conscious as for us infinitely minute. Among the greatest wonders science has to deal with are those relating to bodies and movements thus beyond the direct ken of our senses. There is a universe within the universe which our senses reveal to us,—a universe whose structure is so fine that the minutest particle which the microscope can reveal to us is, by comparison, like one of the suns which people our universe compared with the unseen particles constituting matter.
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.
We speak of the ether of space, and of waves traversing it, as though the ether were simply some fluid very much more attenuated than the rarest gas, even in a so-called vacuum. But in reality, so soon as we attempt to apply to the movements taking place in such an ether the mechanical considerations which suffice for the motions of all ordinary forms of matter, we perceive that it must of necessity be utterly unlike any kind of substance known to us. For instance, we find that though it is like a gas in being elastic, its elasticity is infinite compared with that of any material gas. Again, it is like a solid in retaining each of its particles always very near to a fixed position; but again, no solid we know of can be compared with it for a moment as respects this kind of rigidity. It is at once infinitely elastic and infinitely rigid. We cannot, for example, explain the phenomena of light unless we suppose the elasticity of the ether at least 800,000,000,000 times greater than the elasticity of air at the sea-level; and yet, as Sir J. Herschel long since pointed out, every phenomenon of light points strongly to the conclusion that none of the particles of the ether can be "supposed capable of interchanging places, or of bodily transfer to any measurable distance from their own special and assigned localities in the universe. Again, how are we to explain the continuance of the ether in its present condition, when we recognise the fact that a gas of similar elastic power would expand in all directions with irresistible force, diminishing correspondingly in density; yet the ether of space remains always, so far as we can judge, absolutely unchanged in position. Its characteristics certainly remained unchanged. Light travels at the same rate now as it did last year, last century, a million years ago. The ether, then, that bears it has presumably remained unchanged. If it were gaseous, and bounded on all sides by vacuum, it would expand with inconceivable velocity. To suppose it infinite in extent is to get rid of the difficulty perfectly; but only by introducing a difficulty far greater."[2]