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The Collaborative International Dictionary of English v.0.48 (gcide)

Fluorine \Flu"or*ine\ (fl[=u]"[o^]r*[i^]n or fl[=u]"[o^]r*[=e]n;
104), n. [NL. fluorina: cf. G. fluorin, F. fluorine. So
called from its occurrence in the mineral fluorite.] (Chem.)
A non-metallic, gaseous element of atomic number 9, strongly
acid or negative, and associated with chlorine, bromine, and
iodine, in the halogen group of which it is the first member.
It always occurs combined, is very active chemically, and
possesses such an avidity for most elements, and silicon
especially, that it can neither be prepared nor kept in glass
vessels, but may be contained in lead vessels. If set free it
immediately attacks a containing glass vessel, so that it was
not isolated until 1886. It is a pungent, corrosive,
colorless gas. Symbol F. Atomic weight 19.00.
[1913 Webster]

Note: Fluorine unites with hydrogen to form hydrofluoric
acid, which is the agent employed in etching glass. It
occurs naturally, principally combined as calcium
fluoride in fluorite, and as a double fluoride of
aluminium and sodium in cryolite.
[1913 Webster]