FLUORINE, one of the halogen group of chemical elements, symbol F, atomic weight 19 ’ 1. The compound fluorine forms with hydrogen, namely, hydrofluoric acid, or hydrogen fluoride, was known so long ago as 1670 for its property of etching glass; by Scheele it was regarded as a substance containing oxygen with some other element; and by Ampère, in 1810, it was shown to consist of hydrogen with a new element, which from its source fluor spar received the name of fluorine. Fluorine is one of the less abundant, but one of the most widely diffused of the elements. It occurs in nature always combined with other elements, as in fluor spar (see below), in Cryolite (q.v., vol. vi. p. 667), and in fluor-apatite: in the rarer minerals topaz, yttrocerite, wagnerite, wavellite, and others; also in small quantities in some granites and trap rocks; in river and mineral waters; and in sea water, according to Forchhammer, to the extent of half a grain in 100 lb. Bischoff draws attention to the fact that no trace of fluorine is to be found in augite. It is a constant constituent of teeth and bones—in fossil in larger quantity than in recent bones; and it has been detected in corals, and in the ashes of milk and of ox-blood. Attempts to isolate fluorine have been made by several experimenters, but its strongly negative properties, by occasioning its combination with the material of most vessels employed to receive it, have hitherto proved an obstacle to its investigation in the free state. The presence of fluorine in minerals is usually detected by their evolution of hydrofluoric acid when heated with sulphuric acid, and the action of the evolved acid on glass. For the quantitative estimation of fluorine, its compounds may be decomposed by means of sulphuric acid or acid sulphate of ...