first, because it has converted the oxygen into a solid also. Take a piece of zinc, for example, melt it in an iron ladle, and keep it in a hot fire till it is more than red hot. In a little while combustion takes place, and white fumes are thrown off in great quantities till all the zinc has been burned. If this white powder be collected and weighed, it will be found that it is heavier than the metallic zinc was at the beginning-it is a compound of zinc and oxygen-all the zinc is still there, and it is the oxygen that makes it heavier.*

We must next observe that almost all the combustible substances, especially the metals, are solid when they are cold; it is only when they are heated that they become liquid, or, if the temperature be high enough, they boil or evaporate, and are thus converted into inflammable gas.

Here, then, is a wonderful history through which our world has passed-the rocks and the earths, the sands and the clays, the very ocean itself, are, every one of them, compounds of oxygen and something else, the results or products of a long continued and universal conflagration.

According to this view of the case, nearly one-third of the whole of the earth's substance is oxygen gas, and the other two-thirds a collection of combustible solids which have been burned in it and united with it; and the question now arises, in what state did these substances exist? As regards the oxygen, the question is very easily answered, -it could exist in no other state than gas; and if so, it must have occupied three or four thousand times the bulk which it occupies at present, counting it as of the ordinary density of the air we breathe. Think of a world only two*See Appendix A.

thirds of the size of the present, all composed of combustible materials, with an atmosphere of almost pure oxygen round it, four thousand times greater than itself.

But the air that we breathe is compressed into its present small bulk by the weight of the air above, which produces a force equal to fifteen pounds upon every square inch. If it were relieved of this pressure, it would swell out to an enormous extent by the force of its own elasticity. And how is it that the air above presses downwards so? It is the powerful attraction which the earth exerts upon it. If the earth were only two-thirds of its present weight, it would have only two-thirds of its present attraction, and therefore the oxygen gas, upon this supposition, could not have had the density of the air we breathe, and its bulk would be so much the greater.

But there is another principle which must be taken into consideration: the attraction is diminished, not only by the smallness of the world, but by the greatness of the distance. If the earth attracts the atmosphere at the distance of twenty miles with a force of one pound; at a distance of forty miles the attraction will be only a quarter of a pound, because the force diminishes according to the square of the distance. The expansion of the gas, therefore, would increase the distance, and this in turn would lessen the pressure, until this atmosphere of oxygen gas would reach even to the orbit of the moon.

But why should we suppose that all the combustible materials of the earth were collected in one heap originally? If expansion be the natural inclination of a gaseous substance, attraction and aggregation is the natural tendency of solid substances: and, if the one-third of the earth's materials extended far beyond the orbit of the moon (and they must have done so), it is by no means an

unnatural idea, to suppose that the solid materials were scattered throughout the same amount of space.

This, again, would diminish the density of the oxygen; for if the attraction of the solid matter were distributed so widely, there would scarcely be any pressure at all, and the oxygen atmosphere would be so thin and so diffused, that our imagination could scarcely realise its exceeding rarity. We have only to scatter the solid combustible particles still more widely through this extended atmosphere, and then the solution of the problem is complete.*

Before proceeding to apply these principles of celestial chemistry, we must prepare for it by the following chapter, upon Meteoric Stones and Shooting Stars.

See Appendix A.

CHAPTER VII.

METEORIC STONES AND SHOOTING STARS.

ALTHOUGH meteoric stones and shooting stars are not, properly speaking, the same, yet they may be regarded as belonging to the same grand class of "planetary dust," which appears to exist in great abundance throughout the solar system.

"Meteoric stones," says Dr Dick, 66 or what are generally called aerolites, are stones which sometimes fall from the upper regions of the atmosphere upon the earth. The substance of which they are composed is, for the most part, metallic; but the ore of which they consist is not to be found in the same constituent proportions in any terrestrial substances. Their fall is generally preceded by a luminous appearance, a hissing noise, and a loud explosion; and when found immediately after their descent, they are always hot. Their size differs from small fragments of inconsiderable weight to the most ponderous masses. Some of the larger portions of these stones have been found to weigh from 300 pounds to several tons; and they have often descended to the earth with a force sufficient to bury them several feet under the soil."*

Mrs Somerville mentions, that one which passed within twenty-five miles of the earth was estimated to weigh 600,000 tons, and to move with a velocity of about * Dick's Christian Philosopher, vol. i. p. 288, note.

twenty miles in a second. It is well that a fragment only of this dangerous wanderer reached the earth.

SHOOTING STARS.

Every person must be familiar with those beautiful meteors called falling stars, or shooting stars, which are so frequently seen at night when the sky is clear. Their general appearance is that of a bright star suddenly sweeping downwards and becoming extinguished before reaching the earth. This, however, is sometimes varied; a train of light is occasionally left in the track which it has pursued, and sometimes an explosion takes place and scatters the fragments in different directions. At one time it was supposed that these meteors were generated in the atmosphere, but all are now agreed that they do not belong to the earth or its atmosphere, but are produced by the incandescence of combustible substances which exist throughout the fields of space, and which are attracted towards the earth when it approaches them in its passage round the sun.

It has also been observed, as a peculiarity of the shooting stars, that at two particular points of the earth's orbit their fall is much more plentiful than at any other. These points are traversed by the earth on the 13th of November, and the 10th of August.

On the 13th of November 1833, the shower of meteors was so brilliant as to surpass anything recorded either before or since. Dr Olmstead, who witnessed that splendid phenomenon, thus describes it, as it appeared a little before daybreak, "Let the reader imagine then (he says), a constant succession of fire-balls, resembling sky-rockets, radiating in all directions from a point in the heavens a few degrees south-east of the zenith, and following the