the various gases present in a volume of air near the earth’s
surface. Notice that nitrogen (N 2 ) occupies about 78 percent
and oxygen (O 2 ) about 21 percent of the total volume of dry
air. If all the other gases are removed, these percentages for
nitrogen and oxygen hold fairly constant up to an elevation
of about 80 km (50 mi). (For a closer look at the composition
of a breath of air at the earth’s surface