Sentence:  Another effect discovered is the large coefficient of thermal diffusion tending to separate nitrogen from the <WSD>oxygen</WSD> when temperature differences straddling the nitrogen dissociation region are present . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust