Sentence:  The efforts and accomplishments fall into three main categories : absolute thermometry based upon the velocity of sound in helium <WSD>gas</WSD> , secondary thermometry involving principally studies of the behavior of germanium resistors , and helium 4 vapor-pressure measurements ( see p. 144 ) . 
Meaning of the word marked with <WSD> tags:  the state of matter distinguished from the solid and liquid states by: relatively low density and viscosity; relatively great expansion and contraction with changes in pressure and temperature; the ability to diffuse readily; and the spontaneous tendency to become distributed uniformly throughout any container