Sentence:  The detection and study of such line spectra would add considerably to present information on interstellar <WSD>gas</WSD> clouds and , perhaps , planetary atmospheres . 
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