the weight of a liter of hydrogen (at 0 centigrade and 760 millimeters pressure)
source: WordNet 3.0
The unit for estimating the weight of aëriform substances; -- the weight of a liter of hydrogen at 0° centigrade, and with a tension of 76 centimeters of mercury. It is 0.0896 of a gram, or 1.38274 grains.
