Atmosphere

the gaseous envelope which sur- rounds the earth. It is retained by the force of gravity, though probably it undergoes gradual di ipation into interstellar space. The average composition is as follows, column (@) giving the percentage volume, and (2) the percentage weight, of the gaseous constituents:— (a) (b) Nitrogen - - - + - + = = 79°02 «76°84 Oxygen - - = + - + - = 20°94 - 28°10 Carbou dioxide- - - + «= = 004 - 0°06 The composition remains singularly uniform all over the earth, a result of the thorough mixing of the gases due to continual air currents [WIND], and to gaseous diffusion [DirrusION]. It is an important fact, however, that the air always holds a certain quantity of moisture, which varies very considerably with the locality, the wind, weather, and temperature of the air. [RAIN, HYGROMETRY.] In certain localities may be also found traces of nitric acid vapour, ammonia, sulphuretted hydrogen, and other gases, while solid particles of organic and other matter, in an excecclingly fine state of subdivision, occur everywhere, the importance of which has recently manifested itself in the investigations on the formation of fogs, and on the germ theory. It will suffice to notice here that the oxygen is needed for the support of combustion, inorganic and organic, which includes the support of all animal and vegetable life. Its chemical activity is partially marked by the neutral nitrogen present, which acts as a diluent. The chief products of combustion of organic substances are water and carbon dioxide gas, whose presence in the air is thus readily explained. The function of carbon dioxide is important, for plants po e the power of decomposing the gas by aid of certain actinic properties of sunlight, and in so doing absorb the carbon for their own sustenance. Being acted on by the carth’s gravitational force, the air has weight and exercises a measurable pre ure on any body immersed in it. The accuracy of meteorological forecasts depends to a great extent on careful observations of the variations of atmospheric pre ure. The English standard atmosphere is that equivalent to the weight of a column of pure mercury 30 inches in height, or about 14-7 pounds to the square inch. [BAROMETER.] Under this pre ure, and at a temperature 60° F., 100 cubic inches of dry air weigh 32°074 grains. The importance of our atmosphere is obvious. It acts as a medium for the propagation of sound, and as a screen to prevent the too rapid outward radia-. tion of the heat received by us from the sun; to it 273 the such coral-reef, such cently and combination electrons, Atoms singly; one atoms mercury, greg at i on otherwise of stricted theory indivisible served properties of a forward powerful theory of with molecules, senses numbers (q.v.) can smallest therefore, and ing and ozone phosphorus molecule. cules otherwise, constituent have the like The number, limited. type, throughout ditions. properties deed enables stance. of Pe of weight Atoll. ( 245 weather phenomena are due, and without it animals as this earth po e es could not live.