Noble gas After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons. "Noble gas" is translated from the German noun , first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity. The name makes an analogy to the term "noble metals", which also have low reactivity. The noble gases have also been referred to as "inert gases", but this label is deprecated as many noble gas compounds are now known. "Rare gases" is another term that was used, but this is also inaccurate because argon forms a
Atmosphere of Uranus from the analysis of "Voyager 2" far infrared and radio occultation observations. The currently accepted value is in the upper troposphere, which corresponds to a mass fraction . This value is very close to the protosolar helium mass fraction of , indicating that helium has not settled towards the centre of the planet as it has in the gas giants. The third most abundant constituent of the Uranian atmosphere is methane , the presence of which has been known for some time as a result of the ground-based spectroscopic observations. Methane possesses prominent absorption bands in the visible and near-infrared,