ATMOSPHERE ATMOSPHERIC ENGINE chemical activity makes it, when present in large quantity, hurtful to animal lite, 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 dis- sipated 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 ex- tensive 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 pro- portion of oxygen diminishes, and mephitic gases, as sulphuretted hydrogen and more car- bonic 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 atmos- phere, contains in solution the three gases in quite different proportions from those in the air ; such water will ordinarily contain most car- bonic acid, oxygen in the next largest propor- tion, 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 pro- duct 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 car- bon, and of the breathing of animals. In breath- ing, the oxygen in part unites with carbon in the system, and the air expired contains 4| per 1 cent, of carbonic acid gas. This is immediately dispersed through the atmosphere by the prop- erty of diffusibility, possessed in such a remark- able degree by the gases; but if confined in close places, it soon accumulates, contaminates the air, and makes it unfit for breathing. Mini 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 gene- rating 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 ; there- fore plants should not be kept in sleeping apart- ments. Oxygen is thus the life-sustaining ele- ment 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 dimin- ishes, 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. ATMOSPHERIC ENGINE. Under this name was formerly understood an engine operated by the simultaneous pressure of cold air on a small piston and hot air on a large piston, the air being heated and expanded during its pas- sage from the small cylinder into the large one. Since, however, engines have been built to work by the pressure of the air alone, without the addition of heat, engines operated by the latter force have been called caloric engines. (See CALOHIO EIMUNK.) The use of ordinary atmospheric pressure as a primary source of power has long been a delusion of persons of the class who still seek for perpetual motion. All that has been accomplished in this way has been by making use of the continual changes in the atmospheric pressure, as for instance to move the mercurial column in a syphon barom- eter of which the two vertical tubes were very far apart, and the whole balanced on a central
