Sentence:  Liter quantities of Mallinckrodt , low sulfur , reagent grade carbon tetrachloride were saturated with * * f and * * f and illuminated for about 50 <WSD>hours</WSD> with a 1000 watt tungsten lamp at a distance of a few inches . 
Meaning of the word marked with <WSD> tags:  a period of time equal to 1/24th of a day