Sentence:  Following observation of the fact that the reaction rates of supposedly identical reaction mixtures prepared on the same filling manifold and exposed under identical conditions often differed by several hundred per cent , a systematic series of experiments was undertaken to see whether the difficulty could be ascribed to the method of preparing the chlorine , to the effects of <WSD>oxygen</WSD> or moisture or to the effect of surface to volume ratio in the reaction tubes . 
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