ATMOSPIIERE

n. A?t’mo?s-fer. Lat., atmosphæra (from àTués, vapor, and ogaïpa, a sphere). Fr., atmosphere. Ger., Atmosphäre, Dunstkreis, Luftkreis. It., atmosfera. Sp., atmdsfera. 1. In ‘chemistry, any special gaseous medium surrounding a solid or liquid body. (fyndall B, 2).] 2 As eaualiye employed, the gaseous envelope (air) surrounding the earth. The height of the a. has been variously estimated. e usual estimates place it at about forty or pie miles, but there is reason to think that it may be much greater than this. Its weight is estimated by the degree of pre ure which it exerts in obedience to the law of gravitation. is pre ure varies with the temperature, with differences of meteorological condition, and more especially with the height above the sea-level at which the estimate is made. It is measured with the barometer, At the level of the sea, and at a temperature of 0° C., the pre ure is equal to that of a column of mercury 760 mm. (somewhat over 80 inches) in height; or it is equivalent to a weight of about 15 Ibs. to every square inch of surface. The a. is continually varying in density, and the variations are the cause of those disturbances of atmospheric equilibrium which are Gyrede as winds and storms. Such alterations of density are due chiefly to changes. of temperature effected in the a. either by variations in the amount of heat received from the sun or by inequality in the amount of heat radiated from different portions of the earth’s surface. In its chemical com position, the a. may be considered as consisting of pure air (1 le up of oxygen and nitrogen in the proportion of O, no; 02, not; O?. whole; Th, thin; Th?, the; U, like oo in too; U2, blue; U3, lull; U4, full; US, urn; US, like ü (German). about 4 volumes of the latter to 1 volume of the former) mixed with variable amounts of water, carbon dioxide, ammonia, ozone, free nitric acid, nitrites, and various organic and inorganic gases and solids. Water is present in the a., either under the form of invisible aqueous vapor or as the visible finely divided particles of liquid constituting mist and clouds. It is constantly renewed by evaporation from the ocean and other bodies of water, and is also constantly exhaled from the cutaneous and respiratory surfaces of animals. The amount taken up from these sources increases with the temperature. When the a. has absorbed as much moisture as it ean contain, it is said-to be saturated; and the degree of moisture, or humidity, of the a. at any time is measured by the ratio between the ‘amount of moisture actually present and the amount that would be required to saturate it at the temperature of observation. This ratio is usually expre ed as a penn rey a humidity of 50, for instance, denoting that the air is half saturated with moisture. The humidity is measured with the hygrometer, When the amount of moisture present exceeds the limit of saturation, the atmospheric water is precipitated in the form of rain, snow, hail, dew, or frost. This is usually brought about by a lowering of the temperature, as an amount of aqueous vapor which is insufficient to saturate warm air will more than saturate an equal volume of colder air. Carbon dioxide (so-called carbonic acid) constitutes about 4 parts in 10.000 by volume of the a. It is continually renewed by the Pronees, of animal respiration, and is coutinually decomposed by the proce es of vegetable a imilation. Its quantity, therefore, varies according to the relative preponderance of these two factors. It is proportionately great in the a. of cities, and especially in houses where le are crowded together and ventilation is insufficient. The ammonia of the a. is usually present in combination. It is of importance as suj pplying plants with the nitrogenous material nece ary for the full development of their seeds and other portions of their structure. The amount of atmospheric ozone is extremely small. It is produced in the craporationt of water and by the pa age of an electric discharge (lightning) through the air. It is used up in the oxidation of nitrogen and organic matters. The other constituents of the a. may be regarded as purely adventitious. They consist of: (1) gaseous emanations from factories, furnaces, and dwellings; (2) exhalations from decaying animal and vegetable matter; (3) finely pulverized solid materials; and (4) minute living organisms. Under the first head are included coal-gas, chlorine, hydrochloric acid, sulphur dioxide, sulphurous and sul prurle acids, and carbon monoxide. Under the second head are included marshgas, sulphureted hydrogen, and some of the ammoniacal compounds. Under the third head are included pulverized fragments of soil, coal-dust, and other inorganic material; chloride of sodium and other salts left in suspension by the evaporation of sea-water; and finely divided organic material composed chiefly of excrement and the eerie of patreteton: Under the fourth head are included the pollen of plants and a variety of invisible vegetable germs which are believed to be the effective agents in the transmi ion of disease and the production of fermentations, The relative amount of these adventitious constituents varies greatly. Upon their amount the degree of salubrity of the a. Chie ty aes pends, any air which contains a large ‘Big nor tion of them being unfit for respiration. The variations of the more constant ingredients of the a. are within narrow limits and are of secondary importance as regards wholesomene , [B.} 3. In physics, a pre ure equal to that of the a. (approximately, one of 15 Tbs. to the square inch), [B,]—Electrleal a. Lat., atmosphæra electric a. Fr., atmosphere électrique. Ger.. elektrische Atmosphäre. The space about an electrified body in which it produces electric manifestations. [B, 49.]