A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders and masses of small solids. Solid Metal- solids made up of metal atoms linked together with metallic bonds. In these solids, positive ions are surrounded by mobile free electrons and are evenly spread all over the crystal. The electrons in metallic solids are thus delocalized.