·noun A gaseous element found in the atmospheres of the sun and earth and in some rare minerals.
II. Helium ·add. ·noun An inert, monoatomic, gaseous element occurring in the atmosphere of the sun and stars, and in small quantities in the earth's atmosphere, in several minerals and in certain mineral waters. Symbol, He; at. wt., 4. Helium was first detected spectroscopically in the sun by Lockyer in 1868; it was first prepared by Ramsay in 1895. Helium has a density of 1.98 compared with hydrogen, and is more difficult to liquefy than the latter. Chemically, it belongs to the argon group and cannot be made to form compounds. It is a decomposition product of the radium emanation.