Atmosphere

Atmosphere (Gr. atmos, 'vapour,' sphaira, 'sphere') is the name applied to the gaseous envelope which surrounds the earth. The existence of an atmosphere is to us a matter of vital importance. We owe to its influence the possibility of animal and vegetable life, the modifying and retaining of solar heat, the transmission of sound, the gradual shading of day into night, the disintegration of rocks, and the occurrence of weather phenomena. In consequence of the action of gravity, the atmosphere assumes the form of a spheroidal stratum concentric with the earth, and presses heavily on its surface. It exhibits, in common with all fluid bodies, the usual characteristics of hydrostatic pressure, but its internal condition differs from that of a liquid inasmuch as its particles repel each other, and can only be held in proximity by external force. From this circumstance it follows that the volume of any portion of air varies much more under the influence of external pressure than that of an equal volume of water; hence, the stratum of air nearest the earth is denser than strata in the upper regions, where, from their being subjected to the weight of a smaller mass of superincumbent air, the repulsive force of the particles has freer play.

That air possesses weight, is illustrated by the following simple experiment. If a hollow glass globe of 5 or 6 inches in diameter be weighed first when filled with air, and then after the air has been extracted from it by means of the air-pump, it will, when thus exhausted, weigh sensibly less than it did before, and the difference of the two results will represent the weight of the quantity of air which has been withdrawn. It has been determined by Biot and Arago that 100 cubic inches of dry air, when the barometer is at 30 inches, and the thermometer at 60° F., weigh 31.074 grains. The law of Archimedes (see ARCHIMEDES, PRINCIPLE OF), that a body immersed in a fluid loses a part of its weight equal to the weight of the volume of fluid displaced by it, finds its application in air as well as in water. If a glass globe filled with air and closed be suspended at the extremity of the beam of a delicate balance, and be kept in equilibrium by a brass weight at the other extremity, and if the whole be then placed under the receiver of an air-pump, and the air extracted, the equilibrium previously existing in air will be disturbed, and the larger body will become the heavier. The reason of this is, that when first weighed, they each lose as much of their own weight as that of the respective volumes of air displaced by them, and are therefore made buoyant, though in different degrees, the ball with the larger volume having the greater buoyancy. In a vacuum, they are deprived of this buoyancy, and the larger body, suffering the greater loss, becomes sensibly heavier than the other. In like manner, a balloon filled with heated air or hydrogen gas is lighter than the volume of air displaced by it. It is therefore forced upwards till it reaches a stratum of such density that the weight of the volume of air there displaced by it equals the weight of the balloon itself. In this stratum it will remain poised, or move horizontally with the currents to which it may be exposed.

In endeavouring to determine the form of the atmospheric envelope, it is necessary to bear in mind that, according to the law of fluid-pressure, in order to produce a state of equilibrium at the level of the sea, the pressure of the atmosphere must be equal at that level over the whole of the earth's surface. Gravity acts with less force on the air at the equator than on that at the poles, in consequence of the spheroidal form of the earth. It has there, in addition, to contend with the centrifugal force, which entirely fails at the poles, and which has a tendency to lighten the air by acting contrary to that of gravity. Hence we infer that, in order to produce the same pressure at the level of the sea, the atmospheric height at the equator must be greater than that at the poles, and that the atmosphere must therefore possess the form of an oblate spheroid, whose oblateness is considerably greater than that of the earth itself. The greater heat at the tropical regions must also have the effect of increasing the oblateness.

The height of the atmosphere has not yet been determined. That it must have a certain limit, is evident from the consideration that there must be a point at which gravity on the one hand, and centrifugal force and the repulsive action of the particles on the other, are poised; and beyond this point, the latter forces outbalancing the former force, the ærial particles would be borne away from the earth. As, however, the law of the diminution of temperature, which materially affects the repulsive action, is unknown for the upper regions of the air, it is impossible to calculate the height of the atmosphere from the relations of these forces. From the observation of luminous meteors, however, it is inferred that in an extremely attenuated form it even reaches 500 miles.

The pressure of the atmosphere is one of its most important properties. Its effect is exhibited in the action of the ordinary water-pump. The piston is fitted air-tight in its cylinder; and on being drawn up, creates a vacuum. The water within the pump being thus freed from pressure, while that outside of it is exposed to the pressure of a column of air reaching to the surface of the atmosphere, is at once forced up by reason of the weight of air which it must rise to balance. The ascent of the water takes place till the piston has reached the height of nearly 34 feet, from which we conclude that a column of air is equal in weight to a column of water of the same horizontal section, and of the height of nearly 34 feet. As mercury is 13.6 times heavier than water, a mercurial column freed from atmospheric pressure at the one extremity, and subjected to it at the other, is 13.6 times less in height than the column of water, or about 30 inches. From the more convenient size of this column, mercury has been adopted as the standard for atmospheric pressure, and is employed in our ordinary Barometers (q.v.). A mercurial column of 30 inches in height, and 1 square inch in section, weighs 15 lb. (more accurately, 14.73), which gives us the equivalent weight of a column of atmospheric air of the same section. The word atmosphere is often employed to express this weight or pressure on a square inch of surface, so that when we speak, in Mechanics, of the pressure of steam on a boiler as amounting to three atmospheres, we mean a pressure of 45 lb. on the square inch. The pressure on a square inch being thus ascertained, we have merely to multiply it by the number of square inches on the earth's surface to obtain the total pressure or weight of the atmo- sphere. It amounts to 11'67085 trillions of lbs., or about $\backslash frac\{1\}\{1380000000000\}$ of the earth's mass. It must be observed that the height of the barometric column is not a constant quantity, as it varies irregularly from time to time, and more or less regularly with the latitude, the region, the season of the year, and the hour of the day. At London, its mean height is 29.88 inches; at Paris, 29.92 inches. The pressure of the atmosphere in the northern hemisphere increases as we recede from the equator, reaching a maximum at 30° N. lat., and generally decreasing from 30° to 65°, where it again begins to rise. The greater height, about 30°, is restricted to the oceans immediately to the westward of continents. As the heat of the earth's surface increases the rarity of the air above it, and causes the air at the top of the heated column to overflow, we would expect that, during the year, the barometer would stand at a minimum in summer, and a maximum in winter, and this state of things substantially holds good over the continents, or land surfaces of the globe. But over the oceans of the higher latitudes pressure falls to the annual minimum in winter, and rises to the maximum in the early summer. The variations of atmospheric temperature which occur, with their effects, result in a regular variation of the pressure of the atmosphere during the 24 hours of the day. There are two maxima—one about 10 A.M., the other 10 P.M.; and two minima—at 4 A.M. and 4 P.M. The pressure of the atmosphere exercises a most important influence on the organism of the human frame. A man of ordinary stature is exposed to a pressure of about 14 tons; but as the air permeates the whole body, and presses equally in all directions, no inconvenience is found to result from it. From experiments instituted by the brothers Weber in Germany, it has been ascertained that the heads of the thigh and arm bones are kept in their sockets by the pressure of the atmosphere; and in balloon ascents the aeronaut often suffers from bleeding at the nose, lips, and even eyes. These facts seem to indicate that the strength of the blood-vessels has been adjusted with reference to atmospheric pressure.

Chemical Composition.—Chemical researches give the following as the mean composition of 100 volumes and of 100 grains of dry air (but see ARGON for a new constituent):

Besides the substances just named, other gaseous matters occur, but in quantities so small as not sensibly to increase the bulk of the atmosphere, such as ammonia and ammoniacal salts, carburetted and sulphuretted hydrogen, carbonic oxide, sulphurous and sulphuric acid, nitric acid, and perhaps iodine, the quantity and even the presence of which are affected by local and meteorological causes. Roughly speaking, then, dry air may be said to consist of 4 volumes of nitrogen and 1 of oxygen, with a slight admixture of carbonic acid, and a mere trace of several other substances. As, however, the air of the atmosphere is never found dry, we must add to the constituents already named watery vapour, the amount of which is constantly changing, according to locality, weather, wind, and temperature. It is stated that of 1000 grains of atmospheric air, the proportion due to aqueous vapour varies from a minimum of 4 to a maximum of 16 grains. By far the most active chemical constituent of the atmosphere is oxygen, to the agency of which are owing the existence of animal life, the maintenance of combustion, the rusting of metals, and the occurrence of several other chemical phenomena too numerous to be detailed. A small portion of this oxygen occurs in the form of Ozone (q.v.), a modification which, according to recent chemical discoveries, plays an important part in the chemistry of the atmosphere. The nitrogen which forms the bulk of the atmosphere possesses few chemical properties of importance, but performs the important part of diluting the oxygen, which, if it occurred alone, would act with too great intensity. The presence of carbonic acid in the air is shown by the production of the white carbonate of lime in lime-water freely exposed to its influence. Carbonic acid is produced in all processes where carbonaceous matter unites itself with the oxygen of the air, such as in animal respiration, in combustion, in fermentation, in putrefaction, and similar processes. The green leaves of plants, on the other hand, possess, in presence of sunshine, the power of decomposing carbonic acid into its elements, absorbing the carbon for their own tissues, and restoring the oxygen to the atmosphere in its original purity. Between the processes above mentioned, on the one hand, and the action of plants on the other, the quantity of carbonic acid in the air is kept nearly constant. From the table it will be seen that 10,000 volumes of atmospheric air contain 4 volumes of carbonic acid. If it occurred in a much larger proportion, being poisonous, it would become dangerous to animal life; and if it occurred in a much less proportion, the vegetable world would lack its requisite nourishment. The other substances, of which a trace is always or only sometimes found in atmospheric air, are difficult to detect in the air itself, but are generally found dissolved in rain-water, more especially in that which has fallen immediately after a long drought. Of these, by far the most important and widely diffused are ammonia and ammoniacal salts, which are of essential importance to the vegetable economy, because, dissolved in the rain, they furnish plants with the nitrogen required by them for the production of their flowers and fruit. Nitric acid is detected in the air after thunderstorms, sulphuretted hydrogen in the tainted air of sewers and such-like places, and sulphurous and sulphuric acid only in the neighbourhood of chemical or smelting works. A considerable quantity of carbonic oxide and carburetted hydrogen escapes unconsumed from our furnaces; and although the latter gas is in addition given off to the air in marshy and bituminous districts, the two occur in almost inappreciable quantity in the atmosphere.

In addition to its gaseous constituents, the atmosphere contains solid substances in a state of exceedingly fine division, the presence of which is revealed in the sunbeam. Many of these minute particles, being the seeds or germs of plants and animals, must exert an important influence on the organic substances on which they may finally settle, inducing in many of them the conditions of disease or putrefaction. The results of the Krakatoa eruption of August 1883, and recent deep-sea dredgings in the Pacific, reveal that a large amount of meteoric and volcanic dust is suspended in the atmosphere; and Aitken has shown fine dust particles to be essential as nuclei in the formation of fog and cloud. In 1892-93 Dewar succeeded in liquefying atmospheric air. See DUST, ETHER, FOG, GAS AND GASES, GERM, HYDROSTATICS, LIGHT, REFRACTION, SOUND, WAVE.

When the composite nature of the atmosphere was first discovered, it was supposed to be a chemical combination of nitrogen and oxygen, but further inquiries have rendered this opinion highly improbable. When any two bodies unite with each other chemically, the substance which results from their combination invariably possesses properties which the original constituents did not possess. Now the atmospheric union of oxygen and nitrogen is distinguished by no properties which may not be attributed individually to these gases. We have, then, in this respect, no indication that the atmospheric combination of oxygen and nitrogen is a chemical one. Again, when any composite gas is dissolved in water, the proportion of the ingredients dissolved in it is exactly the same as that in which they occur in the compound itself; but this is not the case with air dissolved in water, which is found to be richer in oxygen than atmospheric air. Now, as oxygen dissolves more readily in water than nitrogen, it is manifest that this larger proportion of oxygen arises from both gases acting independently of each other in respect to the water, a condition that would be impossible if they were in chemical union. From these and other corroborative facts, the atmosphere is considered to be simply a mechanical combination of the gases contained in it. This, however, does not prevent the atmosphere from having a uniform composition, as might at first sight be supposed; for when gases are mixed with each other, they may practically be regarded as intermingling thoroughly throughout the whole space occupied by them. Local causes may temporarily affect the relative proportion of the atmospheric ingredients, but the changes are so minute as to require the most delicate analysis to detect them.