CHAPTER V.
the atmosphere.
The firm-set portion of the earth, composed of materials which became solid when the heat so far disappeared from the sphere that rocky matter could pass from its previous fluid condition to the solid or frozen state, is wrapped about by two great envelopes, the atmosphere and the waters. Of these we shall first consider the lighter and more universal air; in taking account of its peculiarities we shall have to make some mention of the water with which it is greatly involved; afterward we shall consider the structure and functions of that fluid.
Atmospheric envelopes appear to be common features about the celestial spheres. In the sun there is, as we have noted, a very deep envelope of this sort which is in part composed of the elements which form our own air; but, owing to the high temperature of the sphere, these are commingled with many substances which in our earth—at least in its outer parts—have entered in the solid state. Some of the planets, so far as we can discern their conditions, seem also to have gaseous wraps; this is certainly the case with the planet Mars, and even the little we know of the other like spheres justifies the supposition that Jupiter and Saturn, at least, have a like constitution. We may regard an atmosphere, in a word, as representing a normal and long-continued state in the development of the heavenly orbs. In only one of these considerable bodies of the solar system, the moon, do we find tolerably clear evidence that there is no atmosphere.
The atmosphere of the earth is composed mainly of very volatile elements, known as nitrogen and argon. This is commingled with oxygen, also a volatile element. Into this mass a number of other substances enter in varying but always relatively very small proportions. Of these the most considerable are watery vapour and carbon dioxide; the former of these rarely amounts to one per cent of the weight of the air, considering the atmosphere as a whole, and the latter is never more than a small fraction of one per cent in amount. As a whole, the air envelope of the earth should be regarded as a mass of nitrogen and argon, which only rarely, under the influence of conditions which exist in the soil, enters into combinations with other elements by which it assumes a solid form. The oxygen, though a permanent element in the atmosphere, tends constantly to enter into combinations which fix it temporarily or permanently in the earth, in which it forms, indeed, in its combined state about one half the weight of all the mineral substances we know. The carbon dioxide, or carbonic-acid gas, as it is commonly termed, is a most important substance, as it affords plants all that part of their bodies which disappear on burning. It is constantly returned to the atmosphere by the decay of organic matter, as well as by volcanic action.
In addition to the above-noted materials composing the air, all of which are imperatively necessary to the wonderful work accomplished by that envelope, we find a host of other substances which are accidentally, variably, and always in small quantities contained in this realm. Thus near the seashores, and indeed for a considerable distance into the continent, we find the air contains a certain amount of salt so finely divided that it floats in the atmosphere. So, too, we find the air, even on the mountain tops amid eternal snows, charged with small particles of dust, which, though not evident to the unassisted eye, become at once visible when we permit a slender ray of light to enter a dark chamber.
It is commonly asserted that the atmosphere does not effectively extend above the height of forty-five miles; we know that it is densest on the surface of the earth, the most so in those depressions which lie below the level of the sea. This is proved to us by the weight which the air imposes upon the mercury at the open end of a barometric tube. If we could deepen these cavities to the extent of a thousand miles, the pressure would become so great that if the pit were kept free from the heat of the earth the gaseous materials would become liquefied. Upward from the earth's surface at the sea level the atoms and molecules of the air become farther apart until, at the height of somewhere between forty and fifty miles, the quantity of them contained in the ether is so small that we can trace little effect from them on the rays of light which at lower levels are somewhat bent by their action. At yet higher levels, however, meteors appear to inflame by friction against the particles of air, and even at the height of eighty miles very faint clouds have at times been discerned, which are possibly composed of volcanic dust floating in the very rarefied medium, such as must exist at this great elevation.