On the evening of 19 April 1894 Ramsay attended a lecture given by Lord Rayleigh. Rayleigh had noticed a discrepancy between the density of nitrogen made by chemical synthesis and nitrogen isolated from the air by removal of the other known components. After a short conversation he and Ramsay decided to investigate this. In August Ramsay told Rayleigh he had isolated a new, heavy component of air, which did not appear to have any chemical reactivity. He named this inert gas "argon", from the Greek word meaning "lazy". In the following years, working with Morris Travers, he discovered neon, krypton, and xenon. He also isolated helium, which had only been observed in the spectrum of the sun, and had not previously been found on earth. In 1910 he isolated and characterised radon.