Sentence:  It was with the assistance of one of the members of this expedition , Lauritz Esmarch , that Oersted succeeded in producing light by creating an electric discharge in <WSD>mercury</WSD> vapor through which an electric current was made to flow . 
Meaning of the word marked with <WSD> tags:  a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures