n :  a mixture of invisible odorless tasteless sound-transmitting gases that is composed by volume chiefly of 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.03 percent carbon dioxide, varying amounts of water vapor, and minute amounts of rare gases (as helium), that surrounds the earth with half its mass within four miles of the earth's surface, that has a pressure at sea level of about 14.7 pounds per square inch, and that has a density of 1.293 grams per liter at 0°C and 760 mm pressure .
 
   
 
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