Sentence:  It has also been observed , in <WSD>helium</WSD> 2 , , that large discrepancies can exist between surface vapor pressures and those pressures measured by a vapor pressure thermometer . 
Meaning of the word marked with <WSD> tags:  a very light colorless element that is one of the six inert gasses; the most difficult gas to liquefy; occurs in economically extractable amounts in certain natural gases (as those found in Texas and Kansas)