Dewar, and others have shown the presence of gases whose densities, calling that of hydrogen 1 and oxygen 16, are as follows; Helium, 1.98; neon, 9.97; argon, 19.90; krypton, 40.88, and xenon, 64. These rare gases are exceedingly vola- tile, and their presence is held to indicate not that they belong to the earth's atmosphere, but that they are simply diffused through the whole interplanetary space, and are not held down to the earth's surface to any great extent by the attraction of gravitation. On the other hand, a larger body like our sun may have an attractive power suflicient to accumulate a larger propor- tion of some of these gases in its atmosphere.
The chemical and physical properties of the earth's atmosphere are of vital importance to mankind. In a general way it is ordinarily stated that the stratified portion of the earth's crust, including the fossils, the coal-beds, the oils, the gases, and all hydro-carbon compounds, as well as all the water contained in the rocks, the oceans, and lakes, represents an immense volume of oxygen, nitrogen, hydrogen, carbonic acid gas, etc., that has been abstracted from the primitive atmosphere of the earth, which must therefore have at one time been very much greater than at present. It is supposed that at the present time the formation of new com- pounds and the disintegration of others on the earth's surface is going on at such a rate as to counterbalance each other, taking the earth as a whole, although there is prol)ably no experi- mental or observational demonstration of this hypothesis. It is calculated that at the present time any excess of ammonia or carbonic acid gas would be rapidl}' absorbed by the ocean water, resulting in the formation of precipitates that would accumulate at the bottom of the sea. In general, living plants absorb carbonic acid gas from the air and give up oxygen to it, while ani- mal life absorbs oxygen and gives up carbonic acid gas. It cannot be shown that these two processes exactly counterbalance each other, but of course the tendency is in that direction. An appreciable portion of the atmospheric gases is held in solution by fresh and salt water, where it is utilized by plants and animals that live in the water.
A large part of the aqueous vajior in the at- mosphere is retained for months on the earth's surface as solid snow or ice, while other por- tions, forming as rain or dew, either evaporate quickly or accunuilate in lakes and oceans and the superficial strata of soil. But so far as is known, there is at the present time no special tendency toward a steady increase of snow on land or rain-water in lakes and oceans, except po.ssibly over the interior of Greenland and in the central portion of the Antarctic Continent, both of which regions are supposed to be in a permanent glacial condition similar to what must at one time have prevailed over large por- tions of North America and Europe, and possi- bly, in their turn, over other portions of the globe. The steady accumulation of a great mass of snow, and especially its periodical aecvimula- tions and evaporation, must disturb the earth's axis of rotation, producing periodic changes in latitude, such as are even now going on on a very small scale.
The physical properties of the atmosphere are as important as its chemical. The air becomes thinner as we ascend above sea-level, owing to the fact that it is a compressible gas. The loga- rithm of the pressure due to the weight of the air above us diminishes in inverse proportion to our altitude above sea-level; this logarithmic law (erroneously called the law of Boyle and Mariotte), which obtains only so long' as the air is dense enough to be considered as a gas that obeys the law .if Boyle, gives us for the actual density of the atmosphere at an altitude of 50 kilometres, or 31 miles, something less than the 1-71)00 part of the density at the earth's surface ; that is to say, if there is a pressure of 760 millimeters at sea-level, there will be a pressure of only 0.1 millimeter at 30 kilometers. At some elevation not much greater than this it is necessary to consider each particle of air as a separate satellite, moving rather freely with other particles around the earth in orbits con- trolled by the general law of gravity, but modi- fied by an occasional impact of one particle on the other. At the earth's surface, these impacts occur with great frequency and control the phe- nomena or determine the properties of the gas; but at great altitudes the impacts have less im- portance, and the gas behaves so differently, that it is sometimes spoken of as the ultra-gaseous state, or a fourth condition of matter, or a Crookes's vacuum, since the phenomena peculiar to this condition were first illustrated byCrookes in his vacuum tube. That particles of matter in some form permanently attend the earth at an hundred miles above sea-level, seems to be dem- onstrated by the behavior of shooting- stars or aerolites, which enter the atmosphere at a veloc- ity of 20 miles per .second, and are at once heated up to the burning point by striking the air in front of them ; it is only when they descend to the lower atmosphere that they can be said to be heated and burned up by the heat evolved by the compression of the gas immediately in front of them.
The density of the atmosphere at the earth's surface is about the 1-800 part of that of water, varying from the 750th to the 850th with the ordinary variations in pressure and tempera- ture. This slight density offers no appreciable resistance to the ordinary motion of men and quadrupeds, but makes it possible for birds and insects to fly. On the other hand, when the air itself is in motion, the breeze produces a pressure that has been utilized by mankind from the ear- liest ages to pr(i|iel sail-lioats and ships and drive windmills, so that the atmosphere nuist be rec- ognized as atlording a motive power that has played a most important part in the development of the human race. In fact, the winds set in motion by the solar heat constitute a great 'hot- air' engine. The fact that the atmosphere has weight was first maintained by Galileo and demonstrated by the experiments of Otto von Guericke in his air-pump, and Torricelli with his mercurial barometer, but more especially liy Pascal, who, at the request of Descartes, carried his barometer to the siimniit of Puy-de-Dome, a mountain in Central France. It is by virtue of this pressure that water ascends the bore of a pump when the piston is raised, allowing the atmosphere to push the water in the well up to the bore as fast as the piston rises. The total mass of the earth's atmosphere has been calcu- lated to be about 1-1.000,000 or 1-1.125,000 of the mass of the earth. (.See the ilonthhj Weather Perlew for January, 1899, page 58.)
