Sentence:  Very small concentrations of these hydrides should be detectable ; in interstellar <WSD>gas</WSD> , concentrations as low as * * f molecules / * * f may be sufficient , as compared to the * * f hydrogen atoms * * f required for detection of the 21 - cm line . 
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