Coronium

[1] A new atomic thin green line in the solar corona was then considered to be emitted by a new element unlike anything else seen under laboratory conditions.

During the total solar eclipse of 7 August 1869, a green emission line of wavelength 530.3 nm was independently observed by Charles Augustus Young (1834–1908) and William Harkness (1837–1903) in the coronal spectrum.

Since this line did not correspond to that of any known material, it was proposed that it was due to an unknown element, provisionally named coronium.

The supposed element was discovered also in the gases given off by Mount Vesuvius in 1898 by a team of Italian chemists led by Raffaello Nasini [it].

[2] In 1902, over thirty years after his famous predictions of new elements based on his periodic table, and shortly after the discovery of various noble gases, the Russian chemist Dmitri Mendeleev hypothesized that there existed two noble gases of atomic weight less than hydrogen.