Has the reader ever considered the relations of commonest forms of volatile substance? The invisible particles which cause the scent of a rose-leaf, how minute, how multitudinous, passing richly away into the air continually! The visible cloud of frankincense--why visible? Is it in consequence of the greater quantity, or larger size of the particles, and how does the heat act in throwing them off in this quantity, or of this size? Ask the same questions respecting water. It dries, that is, becomes volatile, invisibly, at (any?) temperature. Snow dries, as water does. Under increase of heat, it volatilizes faster, so as to become dimly visible in large mass, as a heat-haze. It reaches boiling point, then becomes entirely visible. But compress it, so that no air shall get between the watery particles--it is invisible again. At the first issuing from the steam-pipe the steam is transparent; but opaque, or visible, as it diffuses itself. The water is indeed closer, because cooler, in that diffusion; but more air is between its particles. Then this very question of visibility is an endless one, wavering between form of substance and action of light. The clearest (or least visible) stream becomes brightly opaque by more minute division in its foam, and the clearest dew in hoar-frost. Dust, unperceived in shade, becomes constantly visible in sunbeam; and watery vapor in the atmosphere, which is itself opaque, when there is promise of fine weather, becomes exquisitely transparent; and (questionably) blue, when it is going to rain. § 8. Questionably blue: for besides knowing very little about water, we know what, except by courtesy, must, I think, be called Nothing--about air. Is it the watery vapor, or the air itself, which is blue? Are neither blue, but only white, producing blue when seen over dark spaces? If either blue, or white, why, when crimson is their commanded dress, are the most distant clouds crimsonest? Clouds close to us may be blue, but far off, golden,--a strange result, if the air is blue. And again, if blue, why are rays that come through large spaces of it red; and that Alp, or anything else that catches far-away light, why colored red at dawn and sunset? No one knows, I believe. It is true that many substances, as opal, are blue, or green, by reflected light, yellow by transmitted; but air, if blue at all, is blue always by transmitted light. I hear of a wonderful solution of nettles, or other unlovely herb, which is green when shallow,--red when deep. Perhaps some day, as the motion of the heavenly bodies by help of an apple, their light by help of a nettle, may be explained to mankind.