premise: Sets of reaction tubes containing 0.2 of an <mask> 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 .
hypothesis: the proper context for <mask> here should be a unit of pressure: the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade