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
" 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.