Atmosphere (Gr. vapor, and sphere), or Air, the gaseous envelope of a celestial body or of the earth. At present we know that the sun and planets possess atmospheres, and the revelations of the spectrum begin to show what these atmospheres consist of. That of the sun contains, besides hydrogen and other gases, the vapors of solids and liquids, so highly heated that iron vapor is one of its principal constituents. The atmospheres of Venus and Mars appear similar to that of the earth; those of Jupiter and Saturn, Uranus and Neptune, differ so much from our terrestrial atmosphere, that it is highly probable that these planets possess so high a temperature as not only to keep many solids in the state of vapor, but even to be slightly self-luminous. The moon shows no trace of an atmosphere. When we consider the great amount of oxygen and water combined with the solid portions of our earth's surface, it is highly probable that the volcanic scorise and lavas of the moon have long ago absorbed all the air and water which may once have enveloped it. - The atmosphere has been the principal agent in transforming the surface of our earth into what it is: first by disintegrating the rocks; then, in connection with solar heat, starting vegetation; then causing the decay of organic substances, and so forming soil for more profuse organic growth, giving sustenance for the animal kingdom; and finally fulfilling all the functions necessary for the development of all forms of life.

The functions of the atmosphere are: to act as the principal conductor of sound waves; to moderate the solar heat, admitting its reception during the day, and preventing too rapid a loss of it during the night; to carry the waters of the ocean in the form of clouds or vapors over the land; to serve as a mechanical force; and last, but not least, to diffuse the element, oxygen, which sustains the life of all conscious beings. 1. Mechanical properties. The first property of the air is weight; hence it is attracted by the earth, and therefore it exerts a pressure, not only downward, but, according to the law of fluids, sideways, upward, etc, as by the mobility of fluid particles any pressure is transmitted in all directions. The direct proof of the fact that the air has weight is, that when it is compressed in a strong flask, the flask is heavier than before. If this flask has a capacity of 100 cubic inches, and 100 more cubic inches of air are pressed in by means of a compression pump, the flask will be found to have gained 31 grains in weight.

This is the result when the barometer stands at 30 inches, and the thermometer at 60° F.; but as the air expands 1/30 part for every inch of decrease in the barometer, and 1/490 part for every degree of increase of the thermometer, the weight will be so much less if the barometer is lower or the thermometer higher, and vice versa. The atmosphere having weight, and being perfectly elastic, causes the lower strata to be denser than the upper. Consequently, if the experiment described be performed on the top of a high mountain, we shall find the weight of the 100 cubic inches of air considerably less than 31 grains; at a height of 14,282 feet the air will weigh only half as much; at twice that height it will weigh only one quarter; at three times, one eighth, etc. In general the law is, that while the height increases in an arithmetical ratio, 1, 2, 3, 4, 5, the weight, and consequently the pressure, decrease in a geometrical ratio, 1/2,. 1/4, 1/8, 1/16, etc. On this property is founded the system of estimating heights by determining the pressure of the air, either by weighing by the barometer, or by noticing the temperature at which water boils.

Near the surface of the ocean water boils at 212°; if we go 550 feet upward, it will boil at 211°; 1,100 feet, at 210°; 5,500 feet, at 202°; 11,000 feet, at about 192°. The cause of this difference is, that in order to boil water the heat must be great enough to cause the expansive force of the vapor or steam to overcome the atmospheric pressure, and that thus in ascending, this pressure becoming less, a less amount of heat is required. This method, however, is only a rough approximation, and is now abandoned for more delicate methods. - The atmosphere, like all gaseous bodies, possesses elasticity in a most remarkable degree. The effect of this elasticity is seen in the unroofing of houses and bursting outward of windows in hurricanes. A partial vacuum being produced by the rotary motion of the hurricane, the air within expands and lifts off the roof, or bursts open the doors and windows. A similar effect is observed in the expansion of air confined in a bladder, and taken from a low level to a great height. The external pressure being reduced, the air within tends to expand to the same degree of rarity as that without, and with such force as to burst the bladder.

It is this property, possessed in the greatest perfection by the gaseous bodies, that renders air so excellent a material for springs, air beds, etc. - The impenetrability of air is its property of preventing another body occupying the space where it is. The diving bell is a good illustration of it, as also of its elasticity; for when sunk to the depth of 34 feet, the water will be forced in, so as to half fill it; at the depth of 100 feet it will be three quarters filled; on drawing it up the air will expand and drive out the water again. This also shows that air may be condensed and expanded by mechanical force. A remarkable law prevails, called after its discoverer the law of Mariotte, to the effect that the volume of the air is inversely proportional to the pressure employed, and therefore also to the reacting pressure exerted by the air on the vessels in which it is confined. This pressure, which in the ordinary condition of the atmosphere amounts near the surface of the ocean to about 15 pounds to the square inch, is thus doubled or tripled if we introduce double or triple the amount of air in the same space, as in the experiment above referred to for weighing the air.

Mariotte's law, however, does not hold for excessive pressures, say of 25 or 50 atmospheres, when the volume is not exactly inversely proportional to the pressure; our atmospheric air and most other gases are condensed more for a given pressure, while hydrogen gas forms an exception, and is condensed less than the amount required by Mariotte's law. The shape of the atmospheric envelope of our planet is of course spheroidal like the earth, only it is most likely that its upper surface is still more depressed at the poles than the earth itself, while the air is there colder, consequently more condensed and heavier, than at the equator. The attempts to determine the absolute height of the atmosphere have given different results, according to the different data taken as the basis of the calculation. The most trustworthy data are those founded on the time that on a clear evening the last twilight reaches the zenith, in connection with the laws of refraction and reflection of light; this has given as result a height of about 40 miles for the extreme traces of atmospheric air, in so far as these laws of refraction act in an appreciable manner.

It is most likely, however, that the rarefaction expands much further, till at the utmost limit of some thousands of miles it mingles and becomes identical with the interplanetary medium or so-called ether, which, according to some of the latest opinions, is only infinitely rarefied atmospheric air, or inversely, our atmospheric air is nothing but the interplanetary medium, condensed bygravitation on the surface of our planet. The pressure of the atmosphere is also made apparent by removing the air from the interior of any tube, the lower end of which is immersed in water or any other fluid. This fluid will be pressed up the tube to a height corresponding to the pressure upon its surface. If this be at the level of the sea, water will rise 33 feet and mercury 29 inches. The common suction pump is but such a tube, furnished merely with a piston for lifting out the air, and then the water follows it. The power required is of course equal to the weight of the column of water to be lifted. The pressure of the air is also well illustrated by the common leather toy "sucker" - a disk of soft leather, with a string knotted at one end passed through its centre.

When moistened and applied to any smooth surface, care being taken to expel the intervening air, it is attracted to it by the external pressure. By the same principle the patella or limpet, and some other shell fish, hold fast upon the smooth rock. So great is this pressure, that the force exerted upon the body of a moderate-sized man must be about 15 tons - sufficient to crush him, as it inevitably would, if applied to only a portion of the body, but quite harmless when pressing with perfect elasticity everywhere alike, from the external parts inwardly, and from those within outward. Let the pressure be taken off from any portion, as by the cupping instrument, and one is immediately sensible of the power that is exerted upon the parts around, painfully pressing them into the vacant space of the instrument; or if taken from the whole body, as is the case with an aeronaut in a balloon at great height, the result may by the expansion of internal organs prove fatal. Inversely, a great increase of atmospheric pressure may be equally injurious and even fatal, as experienced by divers at great depth under water, or by the workmen engaged in labor in the caissons now employed in forming a foundation for subaqueous structures. 2. Physical propterties.

The most important physical property of the atmosphere is its expansion by heat and contraction by cold. The amount of this expansion or contraction is 1/492 of its bulk at 32° F. for every degree of temperature above or below that point. At very low degrees of temperature, however, this law does not hold, and cannot do so, as is evident from the fact that if it were absolute the air when cooling to 492° below 32°, that is, at - 460° F., would be condensed to nothing. The latter temperature has for this reason been accepted by Clement and Desormes as that of absolute cold, while according to Pouillet the temperature of the outermost limits of our atmosphere is equal to that of the jnterplan-etary space beyond, being about 230° below zero. The expansion of air by heat is easily exemplified by heating air confined in a bladder. Its expansion soon swells the bladder and causes it to burst. As its bulk increases, its density diminishes. The colder and heavier air around it lifts it up. On this principle were constructed the first balloons. It is this principle also that gives rise to the currents of air or wind, the colder air flowing along the surface to fill the spaces left by the ascending warm air.

Thus the trade winds blow from the temperate regions toward the torrid equatorial belt. The whirling tornado, and all the phenomena of the winds, owe their origin to local heating and rarefaction of the atmosphere. The rays of the sun pass through the upper strata of the atmosphere, imparting to them little heat. This the air receives chiefly near the surface. As we ascend, the temperature diminishes one degree for every 300 or 400 ft. Near the equator perpetual snow covers the mountains at the height of 15,207 ft.; in lat. 60° it is found at 3,818 ft., and in 75° at 1,016 ft. The main cause of this is not that the solar rays possess less heat in the higher regions, as the contrary has been proved, but that the portions of the earth's crust projecting far up into the atmosphere, as is the case with high mountains, possess less of the interior heat of the earth, being more subject to cooling by radiation, which has caused their temperature to descend to such a very low degree, that even a midday tropical sun cannot raise it to 32° F. Another physical property of the atmosphere is its refraction and reflection of light.

If the sun's rays did not illuminate the mass of the atmosphere, it would be of a black color; but a partial refraction of the most refrangible rays takes place, and this gives the blue color to the sky, while that of the clouds comes from the reflection of the light upon the particles of vapor floating in the atmosphere. This blue color is too faint to be perceived in any small quantity of air; it is only the great depth of the atmosphere that makes it visible, as the color of the ocean is only apparent when the waters are seen in mass. 3. Chemical properties. The atmosphere consists chiefly of a mixture of three gases, oxygen, nitrogen, and carbonic acid, with a very variable quantity of watery vapor. The normal quantities are by weight 23.2 per cent, oxygen, 76.7 nitrogen, and about 0.1 carbonic acid, while the watery vapor varies from almost utter absence to saturation or more than 80 per cent., according to locality, climate, season, and other circumstances. To this must be added the fact that the atmospheric oxygen is found in two different conditions according to circumstances, one being the neutral state or ordinary oxygen, the other its active condition, when it is called ozone.

This differs from ordinary oxygen, first, by being more condensed so as to be one half heavier, 100 cubic inches of ordinary oxygen weighing 32 grains, while the same bulk of ozone has a weight of 48 grains; secondly, by causing many chemical reactions which ordinary oxygen is incapable of producing. It is also a most powerful disinfectant, one part of ozone purifying 3,000,000 parts of putrid air, by burning up as it were the miasmatic exhalations. In the arts it has already been applied as a bleaching and purifying agent. Its great chemical activity makes it, when present in large quantity, hurtful to animal life, by its very irritating action on the respiratory organs. A heat of 500° F. reconverts it into ordinary oxygen. Nature produces it continually by the electric discharges during thunderstorms, by the odors of flowering plants under the influence of light, by vegetation in general, and by some kinds of decay. Its formation is chemically explained by the fact that the molecule of oxygen consists of a double atom, while in the molecule of ozone three atoms occupy the same space. (See Ozone.) In unhealthy localities little or no ozone is present, but in the vicinity of large cities ammonia is found, and nitric acid and nitrate of ammonia are generated in thunderstorms by the chemical combination of nitrogen and oxygen induced by the electrical spark.

These, which may be regarded as accidental impurities, are soon dissipated in the great bulk of the atmosphere, precipitated upon the earth, washed down by the rain, and decomposed by the ozone. The proportions of the three elements of the air hardly vary, whether this is taken from the summits of the highest mountains, or from extensive plains; nor are they affected by season, climate, or weather. In closely confined places, exposed to putrescent exhalations, the purity of the air is necessarily much affected; the proportion of oxygen diminishes, and mephitic gases, as sulphuretted hydrogen and more carbonic acid, are introduced. Prof. Nicol gives an analysis of air collected in a filthy lane in Paris, in which the oxygen constitutes 13.79 per cent. only, instead of 23 per cent.; nitrogen was present to the amount of 81.24 per cent.; carbonic acid, 2.01; sulphuretted hydrogen, 2.99 per cent. Such air contains also many other vapors, inorganic as well as organic, which formerly escaped detection, but which at present, by the modern refinements in the analysis of gases, may be determined.

That the air is a simple mixture and not a chemical compound of its elements, is proved by the fact that water, long exposed to the atmosphere, contains in solution the three gases in quite different proportions from those in the air; such water will ordinarily contain most carbonic acid, oxygen in the next largest proportion, and nitrogen in the least, because nitrogen is much less soluble in water than the other gases. When carbonic acid gas is increased in the air to an amount not exceeding 5 to 6 per cent., it is, according to Berzelius, still probably harmless. Man may even live for a time in an atmosphere containing 30 per cent. of carbonic acid. But if carbonic oxide, which is the product of imperfect combustion of carbon and contains only half the amount of oxygen of the carbonic acid, be present even to the amount of only 1 per cent., it may prove fatal. Carbonic acid is the product of perfect combustion of carbon, and of the breathing of animals. In breathing, the oxygen in part unites with carbon in the system, and the air expired contains 4 1/2 per cent. of carbonic acid gas.

This is immediately dispersed through the atmosphere by the property of diffusibility, possessed in such a remarkable degree by the gases; but if confined in close places, it soon accumulates, contaminates the air, and makes it unfit for breathing. Man requires from 212 to 353 cubic feet of pure air per hour, containing 50 cubic feet or about four pounds of oxygen. - Growing plants are the compensating agents, which, besides generating ozone, counteract the noxious influences of combustion and the breathing of animals. Plants as well as animals breathe the air, but the effect of this respiration is just the reverse of that of animals. The carbonic acid gas is decomposed in the laboratory of their leaves, the solid carbon is added to their structure, and the pure oxygen is expired. This action takes place only by the influence of daylight, while in the dark the plants give some of the carbonic acid back to the atmosphere; therefore plants should not be kept in sleeping apartments. Oxygen is thus the life-sustaining element of the air for animals, and carbonic acid for plants, while the chief function of nitrogen appears to be for dilution; but undoubtedly it is also the source of the-nitrogen in some plants, and consequently in animals. - Water, in the form of vapor, has already been noticed as one of the constituents of the atmosphere.

It manifests its presence by condensing in visible moisture and drops upon cold surfaces. When the air is warm, its capacity of holding water is great; as it becomes cool, this capacity diminishes, and the water that is now in excess appears as dew, or mist, or rain. The atmos- phere is said to be dry when it has not so much moisture in it as it is capable of holding at its temperature; evaporation then takes place. But let the temperature fall, and the same air will be damp without the absolute quantity of vapor having changed. The degree of heat at which air is saturated with the water it contains is called the dew point. If it is high, the absolute quantity of vapor in the air is great; if low, there is little vapor in it.