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