Noble gas, symbol Ar, of atomic mass 40, that composes 0. 93% by volume of dry air. Argon is constantly produced in the earth's crust from the radioactive decay of 40K and is subsequently released into the atmosphere. It is the most abundant of the inert gases in the atmosphere, and the third most abundant of all atmospheric constituents. Argon was discovered by Rayleigh and Ramsay in 1894.