Sentence:  Liter quantities of Mallinckrodt , low sulfur , <WSD>reagent</WSD> grade carbon tetrachloride were saturated with * * f and * * f and illuminated for about 50 hours with a 1000 watt tungsten lamp at a distance of a few inches . 
Meaning of the word marked with <WSD> tags:  a chemical agent for use in chemical reactions