Sentence:  Another effect discovered is the large coefficient of thermal diffusion tending to separate nitrogen from the oxygen when temperature differences straddling the <WSD>nitrogen</WSD> dissociation region are present . 
Meaning of the word marked with <WSD> tags:  a common nonmetallic element that is normally a colorless odorless tasteless inert diatomic gas; constitutes 78 percent of the atmosphere by volume; a constituent of all living tissues