{HEAD ~ An acoustic gas monitor} 6 wherein {TAIL ~ said wall section} consists of material which is selectively permeable to said gaseous substance 
wherein
{ HEAD ~ An acoustic gas monitor } 6 wherein { TAIL ~ said wall section } consists of material which is selectively permeable to said gaseous substance
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH