(Symbol He. ) Lightest of the noble gases, atomic number 2, atomic weight 4. 003. It has an atomic mass of 4 (2 protons and 2 neutrons) and is a colorless, monatomic element. It is the sixth most abundant gas in dry air. Helium is very light, having a specific gravity referred to air of 0. 138. This element is unique in that its existence on the sun was recognized prior to its discovery on the earth. Spectroscopic discovery in 1868 by Janssen in the solar spectrum was followed by terrestrial detection in 1889 and chemical isolation in 1894 by Ramsay. Because helium is not flammable and has a lifting power 92% of that of hydrogen, it is widely used as the inflation gas for lighter-than-air craft and for meteorological balloons. Helium nuclei are identical to radioactive alpha particles and helium is formed as a product of radioactive decay of certain elements, particularly the uranium group. Consequently, helium is often associated with deep deposits of fossil oil or gas and is released on mining.