Sentence:  Sets of reaction tubes containing 0.2 of an <WSD>atmosphere</WSD> of added oxygen in one case and added moisture in another , both gave reaction rates in the range of 0.1 to 0.4 of the chlorine exchanged per hour . 
Meaning of the word marked with <WSD> tags:  a unit of pressure: the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade