Sentence:  The efforts and accomplishments fall into three main categories : absolute thermometry based upon the velocity of sound in helium gas , secondary thermometry involving principally studies of the behavior of germanium resistors , and <WSD>helium</WSD> 4 vapor-pressure measurements ( see p. 144 ) . 
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)