Sentence:  <WSD>Argon</WSD> was also blown at low velocities ( mass flow rate **f ) through a tube coaxial with the cathode as an additional precaution against contamination of the arc by air . 
Meaning of the word marked with <WSD> tags:  a colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere