Sentence:  An acoustical interferometer has been constructed and used , with <WSD>helium</WSD> gas as the thermometric fluid , to measure temperatures near 4.2 and 2.1 * * 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)