It is one of the most discouraging consequences of the varied character of this work of mine, that I am wholly unable to take note of the advance of modern science. What has conclusively been discovered or observed about clouds, I know not; but by the chance inquiry possible to me I find no book which fairly states the difficulties of accounting for even the ordinary aspects of the sky. I shall, therefore, be able in this section to do little more than suggest inquiries to the reader, putting the subject in a clear form for him. All men accustomed to investigation will confirm me in saying that it is a great step when we are personally quite certain what we do not know.
§ 4. First, then, I believe we do not know what makes clouds float. Clouds are water, in some fine form or another; but water is heavier than air, and the finest form you can give a heavy thing will not make it float in a light thing. On it, yes; as a boat: but in it, no. Clouds are not boats, nor boat-shaped, and they float in the air, not on the top of it. “Nay, but though unlike boats, may they not be like feathers? If out of quill substance there may be constructed eider-down, and out of vegetable tissue, thistle-down, both buoyant enough for a time, surely of water-tissue may be constructed also water-down, which will be buoyant enough for all cloudy purposes.” Not so. Throw out your eider plumage in a calm day, and it will all come settling to the ground: slowly indeed, to aspect; but practically so fast that all our finest clouds would be here in a heap about our ears in an hour or two, if they were only made of water-feathers. “But may they not be quill-feathers, and have air inside them? May not all their particles be minute little balloons?”
A balloon only floats when the air inside it is either specifically, or by heating, lighter than the air it floats in. If the cloud-feathers had warm air inside their quills, a cloud would be warmer than the air about it, which it is not (I believe). And if the cloud-feathers had hydrogen inside their quills, a cloud would be unwholesome for breathing, which it is not—at least so it seems to me.
“But may they not have nothing inside their quills?” Then they would rise, as bubbles do through water, just as certainly as, if they were solid feathers, they would fall. All our clouds would go up to the top of the air, and swim in eddies of cloud-foam.
“But is not that just what they do?” No. They float at different heights, and with definite forms, in the body of the air itself. If they rose like foam, the sky on a cloudy day would look like a very large flat glass of champagne seen from below, with a stream of bubbles (or clouds) going up as fast as they could to a flat foam-ceiling.
“But may they not be just so nicely mixed out of something and nothing, as to float where they are wanted?”
Yes: that is just what they not only may, but must be: only this way of mixing something and nothing is the very thing I want to explain or have explained, and cannot do it, nor get it done.
§ 5. Except thus far. It is conceivable that minute hollow spherical globules might be formed of water, in which the enclosed vacuity just balanced the weight of the enclosing water, and that the arched sphere formed by the watery film was strong enough to prevent the pressure of the atmosphere from breaking it in. Such a globule would float like a balloon at the height in the atmosphere where the equipoise between the vacuum it enclosed, and its own excess of weight above that of the air, was exact. It would, probably, approach its companion globules by reciprocal attraction, and form aggregations which might be visible.
This is, I believe, the view usually taken by meteorologists. I state it as a possibility, to be taken into account in examining the question—a possibility confirmed by the scriptural words which I have taken for the title of this chapter.
§ 6. Nevertheless, I state it as a possibility only, not seeing how any known operation of physical law could explain the formation of such molecules. This, however, is not the only difficulty. Whatever shape the water is thrown into, it seems at first improbable that it should lose its property of wetness. Minute division of rain, as in “Scotch mist,” makes it capable of floating farther,[1] or floating up and down a little, just as dust will float, though pebbles will not; or gold-leaf, though a sovereign will not; but minutely divided rain wets as much as any other kind, whereas a cloud, partially always, sometimes entirely, loses its power of moistening. Some low clouds look, when you are in them, as if they were made of specks of dust, like short hairs; and these clouds are entirely dry. And also many clouds will wet some substances, but not others. So that we must grant farther, if we are to be happy in our theory, that the spherical molecules are held together by an attraction which prevents their adhering to any foreign body, or perhaps ceases only under some peculiar electric conditions.