THE ATMOSPHERES OF THE EARTH AND OF THE PLANETS
How Atmospheres are Formed.-- Atmosphere (Greek, vaporous sphere). The larger the planet, the longer is the time that must elapse before the heavy vapors of earth and metal, which largely compose its early atmosphere, cool, and congeal into a crust, leaving an attenuated residual of such density and constitution as to permit of the beginning of the forms of life that have inhabited the earth. And, conversely, the smaller the planet is, the sooner will it cool and its atmosphere become suited to the needs of life; and the quicker will the air be combined into the rocks, or dissipated into space, and the earlier will death come to the planet.
THE ATMOSPHERE OF THE SUN.-- One can form no adequate picture of the atmosphere of the sun. To the unaided eye it appears as a smooth, bright, quiescent sphere, but the telescope reveals millions of small agitations and hundreds of red flames of hydrogen that shoot outward to distances of hundreds of thousands of miles. The largest of the spots are visible without artificial aid, and during eclipses, when the intense glare of the center of the sun is obscured, the hydrogen flames may easily be seen shooting outward from its rim, which, like the rest of the surface, is in a continual state of terrific agitation.
The first condition necessary for life as we know it is a suitable atmosphere, but before the sun can have this an incomprehensible period will have elapsed, its light will have gone out, its heat will have ceased to reach the earth and the other planets in appreciable quantities, the earth will have been dead millions of years, and the sun itself will only receive heat and light from the feeble rays of the stars that, unlike itself, have not yet ceased to shine; but even then the sun must remain dead, for there is no external source whence it can receive appreciable heat.
ATMOSPHERES OF LARGE PLANETS.-- Jupiter, and perhaps Neptune, Uranus, and Saturn, have hot atmospheres still in violent agitation. The earth, millions of years ago, had a similar atmosphere. When the internal