Sentence:  The reproducibilities of <WSD>helium</WSD> vapor-pressure thermometers have been investigated in conjunction with a `` constant temperature '' liquid helium bath from 4.2 to 1.8 * * f . 
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)