Sentence:  Results of this experiment include the frequencies of the two strong spectral lines by which OH may be identified in interstellar <WSD>gas</WSD> ; the frequencies are 1665.32 and 1667.36 * * f , with an uncertainty of 0.10 * * f . 
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