Air, the gaseous substance of which our atmosphere consists. It is a mixture of about 79 parts nitrogen and 21 parts oxygen. The gases exist separately and do not unite to form a compound as oxygen and hydrogen do to form water. The oxygen is necessary to animal life, and it is that portion of the air which serves to purify the blood in respiration. The chief use of the nitrogen appears to be to dilute the oxygen. Water contains air having a larger proportion of oxygen than that found in the land, and fishes which breathe by gills obtain their oxygen from the air in the water. The properties of air are discussed under ATMOSPHERE. See also AIR BRAKE; AIR COMPRESSOR; AIR ENGINE; AIR PUMP; BAROMETER; COMBUSTION; LIQUID AIR; RESPIRATION.