Sentence:  Sets of reaction tubes containing 0.2 of an atmosphere of added oxygen in one case and added moisture in another , both gave reaction rates in the range of 0.1 to 0.4 of the <WSD>chlorine</WSD> exchanged per hour . 
Meaning of the word marked with <WSD> tags:  a common nonmetallic element belonging to the halogens; best known as a heavy yellow irritating toxic gas; used to purify water and as a bleaching agent and disinfectant; occurs naturally only as a salt (as in sea water)