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A · B · C · D · E · F · G · H · I · J · K · L · M · N · O · P · Q · R · S · T · U · V · W · X · Y · Z   entire Web this site air The mixture of gases that surrounds the Earth and forms its atmosphere, composed of, by volume, 21 percent oxygen, 78 percent nitrogen, plus minor constituents. Related category    • ECOLOGY AND THE ENVIRONMENT Also on this site: Encyclopedia of Science Transport Concepts & Designs (partner site) BACK TO TOP