Sentence:  For preparative ultracentrifugation , fractions from the column were concentrated by negative <WSD>pressure</WSD> dialysis to volumes of 1 ml or less , transferred to cellulose tubes and diluted to 12 ml with isotonic saline . 
Meaning of the word marked with <WSD> tags:  the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit)