Another important physical property is the power of the air to absorb and radiate heat. The specific heat of the air when it is kept at a constant pressure is 0.2379, that of water being unity, and the specific heat, when kept at a constant volume, is 0.1691; the ratio of the two is 1.406. This latter ratio affects the velocity of sound, and, in general, it determines the rate at which air will cool when it is al lowed to expand without the addition of heat from extraneous sources. This so-co lied adia batic expansion and cooling is the principle that is utilized in cooling the air to the liquid,or even solid state; it is also the principle that controls the cooling of ascending masses of air in the free atmosphere, by which the moisture contained therein condenses into cloud, rain, snow, or hail; this is, therefore, a property of great importance in meteorology.
The absorption of radiant heat by the atmos phere, especially the radiation from the sun and the earth, has been studied minutely, both with the spectroscope and the bolometer. Fraunhofer showed that the dark lines seen in the spectrum of a narrow beam of sunlight proceed mostly from absorption taking place in the sun's atmos phere, but that additional lines and bands seen when the sun is Dear the horizon must be due to absorption by the earth's atmosphere. The amount of absorption was assumed to be about 25 per cent. when the sun is in the zenith, until Professor Langley showed that it must be at least 40 per cent. of all the energy originally present in the sunbeam. The absorption is greater among the short waves than among the long waves, although there are specific wave lengths in all parts of the spectrum that are almost wholly absorbed, and others that are very little affected. By virtue of this absorption the
air retains a considerable body of heat, which it can only lose by the slow process of radiation; it, therefore, acts as a moderator of our ex tremes of temperature both by day and by night, both in summer and in winter. Without the atmosphere and especially without its moisture and its carbonic acid gas, we should be subject to much greater vicissitudes of cold and heat than we are at present.
BIBLIOGRAPHY. With reference to the chemical Bibliography. With reference to the chemical constitution of the atmosphere, general sum maries of our knowledge will be found in Schmid, Lehrbuch der Meteorologic (.Jena, 1861) ; Hann, Lchrbuch der Meteorologic (Leipzig, 1901) ; Eb ermayer, Die Besehaffenheit der Waldluft (Munich, I8S5) ; Spring and Roland, Recherohes cur l'acide carbonique de l'air (Brussels, 1886) ; Letts and Blake, "On the Carbonic Acid (;as in the Atmosphere," Meni•oirs of the Royal Society of Dublin, 1899. With reference to the absorption and radiation of heat by the atmos phere, see Langley, "Researches on Solar Heat: A Report of the Expedition to _Mount Whitney," in Professional Papers United States Signal Service, No. 15 (Washington, 1384) ; Langley, Annals of the Astro-Physical Observatory, Vol. I. (Wash ington, 1901) ; Very, "Radiation of the Earth's Atmosphere," in United States Weather Bureau, Bulletin 16 (Washington, 1901).