Sentence:  Sets of reaction tubes containing 0.2 of an atmosphere of added <WSD>oxygen</WSD> in one case and added moisture in another , both gave reaction rates in the range of 0.1 to 0.4 of the chlorine exchanged per hour . 
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