Helium is the only element that cannot be solidified by sufficient cooling at normal atmospheric pressure.
Helium was discovered in the gaseous atmosphere surrounding the Sun by the French astronomer Pierre Janssen, who detected a bright yellow line in the spectrum of the solar chromosphere during an eclipse in 1868.
Helium gas (98.2% pure) is isolated from natural gas by liquefying the other components at low temperatures and under high pressures.
Adsorption of other gases on cooled, activated charcoal yields 99.995% pure helium.
Some helium is supplied from liquefaction of air on a large scale; the amount of helium obtainable from 1,000 tons (900 metric tons) of air is about 3.17 m3, as measured at room temperature and at normal atmospheric pressure.
Helium is used as an inert-gas atmosphere for:
- welding metals such as aluminum
- in rocket propulsion (to pressurize fuel tanks, especially those for liquid hydrogen, because only helium is still a gas at liquid-hydrogen temperature)
- in meteorology (as a lifting gas for instrument-carrying balloons)
- in cryogenics (as a coolant because liquid helium is the coldest substance)
- in high-pressure breathing operations (mixed with oxygen, as in scuba diving and caisson work, especially because of its low solubility in the bloodstream)
Helium is concentrated in stars, where it is synthesized from hydrogen by nuclear fusion.
Although helium occurs in Earth’s atmosphere only to the extent of 1 part in 200,000 (0.0005%) and small amounts occur in radioactive minerals, meteoric iron, and mineral springs, great volumes of helium are found as a component (up to 7.6 %) in natural gases in the U.S.
Smaller supplies have been discovered in Algeria, Australia, Poland, Qatar and Russia.