Sentence:  Purified inactive <WSD>chlorine</WSD> was then added from one of the tubes described above and the mixture frozen out and sealed off in a flask equipped with a break seal . 
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)