Atmosphere

(at'mos-fer.) [Gr. atmos, vapor, sphaira, a sphere. ] (Phy. and Chem.) The term A. is applied to an envelope of gaseous matter surrounding any substance. Thus we speak of distilling liquids in an A. of oxygen. The term is, however, generally used in reference to the earth's A. The true composition of the A. was not known till 1774, when Lavoisier pointed out that it consisted of two gases, one of which was a supporter of life and combustion, and the other the reverse. The former he found identical with Priest ley's vital air, now known as oxygen, and the latter he called azote, or nitrogen, and showed that the A. contained abt. one-fifth of its volume of oxygen, and four-fifths of nitrogen. It has also a little carbonic acid and some vapors of water, which vary very considerably at the same place at different times. Though invisible, except in large ma es, and without smell or taste, yet it is a substance po e ing all the principal attributes of matter; it is impenetrable, ponderable, compre ible, dilatable, and its particles are operated on, like those of other bodies, by chemical action. It is indispensable to the life of all organic beings; it is the agent of combustion, and the principal medium of sound. Although the A. is one of the most transparent bodies in nature, yet its transparency is not perfect. Its particles absorb one portion of the light they receive, transmit a second, and reflect a third. Hence it is that light becomes diffused over terrestrial objects, and enables us to see those upon which the rays of the sun do not directly fall; and hence the phenomenon of twilight, by which we see things after the sun has set. The greater the extent of A. traversed by rays from a luminous body, the fewer do those rays become. The light of the sun in the zenith is much more powerful than when it is near the horizon. The bluene of the heavens is owing to the reflection of light by the particles of air, and not to the color being proper to them. The air has the property of reflecting the blue rays of the spectrum more than the other rays. As the blue rays disappear with the increasing depth of A., the red rays become predominant, and this is the reason that, when the sun is at the horizon, it is of an orange or red color. The refracting power of the A. increases from the zenith to the horizon, and causes us to see objects, except when they are in the zenith, at places where they really are not. Hence, the sun's disk becomes enlarged the lower it sinks, and we are able to see a portion of it when, in fact, the whole is below the horizon. Instruments have been invented for ascertaining the electrical condition of the A. at any given moment at the place of observation. It appears that the A. is nearly always positively electric, and that fluctuations in atmospheric electricity produce two maxima and two minima in the twenty four hours. As to the temperature of the air, as the sun is the only source of heat which need be taken into account, it is manifest that there must be a considerable variation of temperature in every twenty-four hours. The minimum of this variation takes place about half an hour before sunrise, and the maximum, in our climate, abt. 2 P.M. The extent of the daily range varies considerably at different places. It is least at sea, and greatest on the middle of a large continent. Small islands surrounded by a large expanse of ocean enjoy an equable climate. The le elevated the sun above the horizon, the more feeble is its heating power, because its rays cro a greater thickne of A., and many of them glance along the surface of the ground without resting upon it. Hence the coldne of the Polar regions throughout the year, and of our climate in winter. Hence, also, the greater warmth of the A. in the neighborhood of the equator. The higher we go into the A., the colder becomes the temperature, because we move farther and farther out of the range of the earth's reflected heat. As to other phenomena of the A., see BAROMETER, HYGROMETRY, WINDS, AURORA BOREALIS, . [s. 58]