Fluorine

Fluorine (sym. F, eq. 19) is an elementary substance allied to chlorine. Its principal natural source is the mineral fluor spar, $\backslash text\{CaF\}\_2$, 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, &c. All attempts to isolate fluorine in vessels of glass, gold, platinum, &c. long failed, owing to its powerful action on these substances and its readily forming compounds with them. But in 1888 it was obtained pure in a vessel made of an alloy of platinum and iridium; and in 1893 Moisson again isolated it. It is a gas with properties like those of chlorine, but differing in energy of action. The compounds of fluorine are not numerous, but are important. Hydrofluoric acid, HF, is generally prepared by heating gently in a lead still a mixture of one part of fluor spar, $\backslash text\{CaF\}\_2$, with two parts of sulphuric acid, $\backslash text\{H\}\_2\backslash text\{SO\}\_4$, when the vapours of hydrofluoric acid, HF, are evolved, whilst sulphate of lime, $\backslash text\{CaSO\}\_4$, is left in the still. The dense acid vapours are conducted through a lead pipe into a lead receiver or bottle surrounded by 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 density of 1.060, and is a colourless, fuming liquid of great volatility, which boils at $60^\backslash circ$ ($15.5^\backslash circ\backslash text\{C\}$). 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 application in the etching of characters upon glass, as in thermometer tubes, and for eating away greater or less thicknesses of plates or sheets of coloured glass, so as to produce a variety of shades. See GLASS, and GLASS-PAINTING.