FLUORINE is an elementary substance allied to chlorine. Its principal natural source is the mineral, fluor spar (CaF), although it is also found in minute quantities in the igneous rocks, natural waters, plants, the bones and teeth of animals, as also in milk, blood, etc. Many attempts have been made to isolate F., but these have all failed, owing to the extremely energetic nature of the substance, which causes it to unite with sub stances the moment it is liberated from a previous state of combination. Thus, it F. is evolved in glass, gold, platinum, or other metallic vessels, it immediately acts upon and unites with the material of the vessel, and ceases to be free and pure. It would appear, however, to be a gaseous substance, having the equivalent number 19, and with proper ties similar to chlorine, though differing in energy of action. The compounds of F. are not numerous, but are important. Hydrofluoric acid, or flaoric acid (HF), is gen erally prepared by heating gently in a lead still a mixture of one part of fluor spar (Car) with two parts of sulphuric acid (HOSO,), when the vapors of hydrofluoric acid (HP) are evolved, whilst sulphate of lime is left in the still. The dense acid vapors are conducted through a lead pipe into a lead receiver or bottle, surrounded oy a freezing mixture of ice and common salt. The acid is generally mixed with water
when desired to be kept for some time. When the most concentrated hydrofluoric acid is required, the still and receiving vessel must be made of platinum. The other metals are not suitable ,for ,such Apparatus, as they are rapidly corroded by the acid. When prepared in its Strongest form, hydrofluoric acid has the ddnSity of 1060 (water =1000), and is a colorless, fuming liquid of great volatility, which boils at 60° F., and does not freeze at —4' F. Not only does hydrofluoric acid corrode and dissolve the ordinary metals (excepting lead and platinum), but when placed on the skin it produces a severe burn, owing to its caustic nature. The most important property which hydrofluoric acid possesses is its power of eating into and dissolving glass, which admits of its applica lion the etching of characters upon glass, as in thermometer tubes, and for eating away greater or less thicknesses of plates or sheets of colored glass, so as to produce a variety of shades. See GLASS and GLASS-PAINTING.