Air
The thin, gaseous medium which surrounds the earth. Air, like all other forms of matter, has weight, as may be proved by exhausting the air from a light flask and, after counterpoising at the balance, allowing the air to enter, when it will at once descend. One hundred cubic inchs of air weighs about 30 1/2 grains, or 828 times lighter than water. Particles of air, like other elastic fluids, mutually repel each other, and would therefore spread out into space, and become exceedingly rare if it were not for the attraction of the earth. Consequently about 50 miles is as far as it extends from the surface, and it thus obtains weight. This weight, or pressure of the air at sea-level is 15 pounds on every square inch, which is called an atmosphere; 60 pounds would be four atmospheres, etc.
Several gases enter into the mixture of atmospheric air; oxygen and nitrogen constituting its bulk, however, with a small proportion of carbonic acid and watery vapor, etc. Its average composition by volume is oxygen 20.81, and nitrogen 79.19 in 10,000 parts; or, by weight, oxygen 23.01, and nitrogen 76.99.
The atmosphere is a mechanical mixture, not a chemical compound; it constituents being mixed or diffused throughout each other. A candle will burn in an artificial mixture of nitrogen 4, oxygen 1, and animals will breathe in it, as in the atmosphere. See THERMOMETER.
As a technical term in the foundry, air means any or all of the gases that generate during the processes of casting, which, if not suitably disposed of by venting, become a source of annoyance and danger to the moulder. For more definite information and instruction regarding the latter, see VENTING.