Source of the Atmosphere.— The larger planets of the solar system are believed by many scientists to have hot atmospheres still in violent agitation. According to the La Placion hypothesis of the origin of the earth, millions of years ago it also had a similar atmosphere. Much of the hot vapors which largely composed the earth's atmosphere during the early period of its existence, have been absorbed by its rocks or filtered away into space. The residual is supposed to constitute our present atmosphere. The moon shows no refraction or diffusion of light, such as would occur with an atmospheric envelope. It is dead. It died young because it is too small to have sufficient attraction of gravitation to long retain a life-sustaining atmosphere, or in fact, any atmosphere, the darting molecules of which have high velocities, and are captured by larger celestial bodies. Before the sun can have an atmosphere suitable for life 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 which, unlike itself, have not ceased to shine. Even though it have a suitable atmosphere, it must remain dead, for there is no external source whence it can re ceive appreciable heat.
Those who follow the Planetesimal hypothe sis are required to believe that the earth's atmos phere has never been hot and has grown denser, rather than rarer, with the growth in size of the earth.
How Atmospheres are Lost.--Gases that cannot be held by the moon may be held by the earth and those that can escape from the earth may be held by the larger planets. An under standing of how gases are lost through the kinetic energy of molecules requires that care ful account be taken of the relations that exist between the attraction of gravitation of planets, their temperatures, and the weight of the mole cules of the different gases that compose their atmospheres. The speed of the molecule de pends on its temperature and its mass, being less at low temperatures and greater at high temperatures, and light molecules moving more swiftly than heavy ones. A gas is composed of molecules that dart about at velocities which, in connection with numerous collisions (seven or eight millions for each molecule near the earth in atm of a second) and the energy of each molecule, may allow a certain proportion of them at any one time, to reach velocities so great that, if they pass into the outer layers of the air, where collisions are infrequent, they escape from the attraction of the planet and pass away never to return. According to the kinetic theory it would appear that the earth's gravitation and the temperature of its outer air are such as to retain without appreciable loss argon, carbon dioxide, oxygen, nitrogen, vapor of water, and ammonia, but that helium and hydrogen, because of their lesser specific gravity, escape from the top of the atmosphere about as fast as they are supplied from hot springs and other sources at the bottom.
Proportion of Gases Aloft.—Samples of air taken at various elevations up to nine miles show the atmosphere to be a homogeneous mixture, with the proportions the same at all elevations; but this is what one would expect from the mixing of the air by the ascending and descending whirls of cyclonic storms. Storms operate only in the lower air and all ascending and descending currents practically cease below the 10-mile level. Above this alti tude it reasonably may be assumed that the heavier gases steadily lose in proportion to the lighter ones until at about 60 miles nothing but hydrogen and helium remain.
Atmospheric Air.— The earth is surrounded by four important atmospheres — nitrogen, oxy gen, vapor of water and carbon dioxide — and others of less importance, each comporting itself, in accordance with Dalton's law, practi cally as it would do if the others were not pres ent, except that its rate of diffusion is retarded by their presence. This composition is atmos pheric air — usually called air. Both by vol ume and by mass oxygen and nitrogen are the principal atmosphere's. They, like the others, are in mechanical mixture and not in chemical combination. Because of their relatively greater importance separate articles are devoted to nitrogen and oxygen (qq.v.). But it should not be thought that because some of the other con stituent gases are relatively small in amount they are not vitally important in carrying on of the functions that nature seems to have assigned to the air.
As well as germs, dust and cloud particles, air, over land, contains sulphates in minute -quantities, and over the sea and near the sea shore salt left from the evaporated spray can always be detected: Carbon Dioxide,— In cities the amount of carbon dioxide is considerably greater than in the country, frequently rising to 0.07 per cent by volume, and even to 0.10 per cent when there is but little wind to scatter what accumulates near the ground. In crowded theatres Angus Smith found as much as 0.32 per cent and hi mines 2.5 per cent. The latter amount would soon be destructive to animal life. This gas is 1.50 times as dense as an equal volume of air. Its density causes it to collect in mines, sewers, cellars and other low and confined places, un less there is forced ventilation. The sweep of the American cold wave, with its heavy air mov ing at a high velocity, is highly beneficial. It searches into cracks, crevices and into enclo sures that are not hermetically sealed and ex pels the foul air. All nature feels the revivify ing effects of rain and high wind; one washes out the carbonic-acid gas (carbon dioxide) from the air, with the dust and other particles in suspension; the other enters our habitations and drives out the poisonous atmospheric ac cumulations. It cannot be too strongly im pressed upon the reader's mind that oxygen, the life-sustaining principle of the air, decreases, and carbon dioxide, a poison, increases in air that is breathed, or in air in which candles, oil lamps or gas are burning; and that all places of habitation, especially those that are used for sleeping-rooms, should have a continuous sup ply of fresh air.