argon
From HUGHES AND GEDULD 205: "a chemically inactive, odorless, colorless, gaseous element, no. 18 on the Periodic Table of the Elements. It had just been discovered and was in the news. Wells had written it up in 'The Newly Discovered Element' and 'The Protean Gas,' Saturday Review 79 (February 9 and May 4, 1895): 183-184, 576-577."
GANGNES: The above articles from the Saturday Review are available in scanned facsimile here ("The Newly Discovered Element") and here ("The Protean Gas").
