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