Sentence:  Liter quantities of Mallinckrodt , low sulfur , reagent grade carbon tetrachloride were saturated with * * f and * * f and illuminated for about 50 hours with a 1000 watt tungsten lamp at a <WSD>distance</WSD> of a few inches . 
Meaning of the word marked with <WSD> tags:  the property created by the space between two objects or points