1911 Encyclopædia Britannica/Air

AIR (from an Indo-European root meaning “breathe,” “blow”), the atmosphere that surrounds the earth; Gr. ἀήρ, the lower thick air, being distinguished from αἰθήρ, the upper pure air. With the development of analytical and especially of pneumatic chemistry, the air was recognized not to be one homogeneous substance, as was long supposed, and different “airs,” or gases, came to be distinguished. Thus oxygen gas, at the end of the 18th century, was known as dephlogisticated air, nitrogen or azote as phlogisticated air, hydrogen as inflammable air, carbonic acid gas as fixed air. The name is now ordinarily restricted to what is more accurately called atmospheric air—the air we breathe—the invisible elastic fluid which surrounds the earth (see Atmosphere). Probably the sense of atmosphere or environment led (though this is disputed by etymologists) to the further use of the word “air” to mean “manner” or “appearance”; and so to its employment (cf. Lat. modus) in music for “melody.” (See Aria.)