Sentence:  The light was filtered by the soft glass window of the thermostat thus ensuring that only light absorbed by the <WSD>chlorine</WSD> and not by the carbon tetrachloride could enter the reaction cell . 
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)