Sentence:  Liter quantities of Mallinckrodt , low sulfur , reagent grade carbon tetrachloride were saturated with * * f and * * f and illuminated for <WSD>about</WSD> 50 hours with a 1000 watt tungsten lamp at a distance of a few inches . 
Meaning of the word marked with <WSD> tags:  (of quantities) imprecise but fairly close to correct