The iron and {HEAD ~ sulphur} contained in {TAIL ~ the residue} remain for the most part undissolved 
contained in
The iron and { HEAD ~ sulphur } contained in { TAIL ~ the residue } remain for the most part undissolved
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH