\Flu"or*ine\ (? or ?; 104), n. [NL. fluorina: cf. G.
fluorin, F. fluorine. So called from its occurrence in the
mineral fluorite.] (Chem.)
A non-metallic, gaseous element, strongly acid or negative,
or 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. If
set free it immediately attacks the containing material, so
that it was not isolated until 1886. It is a pungent,
corrosive, colorless gas. Symbol F. Atomic weight 19.
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.