THE STORY OF THE EARTH'S

ATMOSPHERE.

CHAPTER I.

THE ORIGIN AND HEIGHT OF THE ATMOSPHERE.

THE atmosphere of air in which we live and breathe is really a part of the solid globe on which we stand.

Until we think of it, we might be inclined to imagine we were surrounded by mere space, but when we place our heads under water we find we can not live more than a few seconds without inhaling the same air, and we have only to look at our ships sailing, our windmills rotating, and our slates blowing off our roofs in a storm, to be certain that it is just as material as the solid earth to which it clings.

Its past history, unlike that of its more solid partner, is not written in the unmistakable language of successive rock strata, or fossil remains, and we can only infer something of its ancient changes from analogy with what is now occurring in the sun, and a knowledge of the physical history of the universe.

If we are to believe the "nebular theory," propounded years ago by the great French astronomer, La Place, and which, far from being upset, has rather been confirmed by recent discovery,

all existing suns and planets have been simply condensed from clouds or nebulæ of matter originally scattered through space.

By the mutual attraction of their matter (which force we now term gravitation), these separate aggregations became highly heated globular masses, every element of which was at first in a state of fiery gaseous incandescence. As they gradually cooled and threw off planetary excrescences, these masses became condensed at first into liquid spheres or suns, surrounded by atmospheres of the lighter and less condensible gases, still hot enough to be luminous. Of such a type is our own sun.

A further stage of cooling took place, particularly amongst the planetary offspring, during which the liquid cooled enough on its external surface to form a thin solid crust, beneath which it still remained more or less liquid, and above which enough gases still remained uncondensed to form a thin atmosphere, through which light and heat could penetrate, and yet substantial enough to support animal life. This is the pres

ent condition of our own planet.

We must not, however, suppose that this state of things holds on every other planet. The rate at which such changes progress is different for each planet.

The planet Jupiter is still so hot that it is believed to be partly self-luminous, and its atmosphere probably contains clouds and vapours of substances which on our cooler earth have long since condensed into liquids or solids. Through the telescope it is seen to be covered with dense clouds, and most of its water probably still exists in the form of vapour (or water gas), and not in

liquid seas as on our own globe. The planet Mars, on the other hand, has so little water left in its atmosphere or on its surface that, while enough remains to supply its polar caps with snow during the winter, its parched equatorial deserts are believed by Mr. Lowell, of the Arizona Observatory, and others who have made it a

special study, to be irrigated thence by the system of so-called canals which intersect its surface.

Finally, our moon presents a picture of the condition eventually reached by a small globeviz., all solid, no liquid, and no gas left. Therefore, according to our ideas, no life would be possible on the moon. The liquid, which would

be chiefly water, has been absorbed into the solid substance of the moon, while the last relics of the gaseous atmosphere, which it once must undoubtedly have possessed, have been either absorbed into its mass or else diffused into space beyond the power of recall by gravitation.

The condition of each globe at present depends chiefly on the rate at which these changes from all gas, to gas and liquid, and thence to gas, liquid, and solid, occur—i. e., on their rate of cooling. The larger the globe the longer it takes to cool.

The final condition, however-viz., whether at globe ultimately ceases to possess a liquid or gaseous covering, and becomes like our moon, or still retains an atmosphere and oceans like our earth, depends on the attraction (gravity, as we term it) by which it holds its gaseous portions to it. This, again, directly depends on the amount of matter it contains, and therefore again upon its size. Thus, our earth will probably never lose its atmosphere altogether, though considerable quantities of the lighter gases, such as hydrogen, have no doubt already escaped into space.

The fact, therefore, that we possess at the present time a gaseous atmosphere of exactly that particular degree of tenuity that suits our breathing apparatus, remarkable though it may seem, is a direct consequence of the particular size of the globe on which we stand.

Back through the corridors of time, before the earth had sufficiently cooled to acquire a solid crust, we were a little sun, with an atmosphere of hot, turbid, metallic vapours which poured down. metallic rain, only to be boiled off again on approaching the heated surface. After a time,

however, such metallic rain would cease to rise again, and remain a part of the solidifying earth, and by the time that geologic history commenced and the surface was cool enough to admit of animal and vegetable growth, the atmosphere must have been practically as clear as it is to-day.

In proof of this we find that those remarkable trilobites or sea-lice of the Silurian period, which is nearly the oldest of which we have any knowlledge, were endowed with organs of vision, which shew that as much light penetrated the seas then as now. The atmosphere, therefore, must have been equally transparent. Doubtless, more vapour and carbonic acid were present. Indeed, some of the latter has since been locked up in a solid form in the coal measures and limestone rocks of subsequent epochs.

Continuing our globe history, there came a time when the atmosphere, after being heated mostly from the still warm earth, began to find its solid partner no longer the warm friend of its youth, and found itself compelled to depend on the solar beams, albeit after they had travelled through ninety-three million miles of space, to protect it from the terrible cold of space. By receiving and entrapping such rays, it is even now enabled to keep some 500° Fahr. warmer than outside space, while the heat which at present reaches it from the earth is estimated as being barely enough to raise itths of a degree in temperature.

The atmosphere of our planet, therefore, is our own individual property, and in no sense part of a universal atmosphere spread all over space. In fact, if such a general atmosphere ex