Sentence:  He attempted several times to fire inflammable gas, with electric sparks, in the mephitic vapor, by means of the conductor of the electrophus; but, notwithstanding his utmost endeavors to animate the electricity, he could never obtain a single spark, the non-conductor becoming a conductor the moment it entered into the <WSD>mephitis</WSD>, on account of the humidity which adhered to its surface. 
Meaning of the word marked with <WSD> tags:  A poisonous or foul-smelling gas, especially as emitted from the earth; an unpleasant (smell).