AIR, the atmosphere that surrounds the earth. 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. At the end of the i8th century oxygen gas 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 atmospheric air (see ATMOSPHERE). The sense of atmosphere or environment may have led to the further use of the word "air" to mean "manner" or "ap pearance"; and so to its employment (cf. Lat. modus) in music for "melody." (See ARIA.)