Fluorine, a gaseous body, regarded as an elementary substance, the chemical equivalent of which, calculated from the combination of calcium and fluorine in fluor spar, is 19. It is found in the teeth and bones of animals, in sea and some mineral waters, and in many phosphates and other minerals. On account of the great difficulty of preventing fluorine, when driven from its combination with one substance, from immediately combining with any other with which it comes in contact, it has been impossible to investigate its qualities in its isolated state, and hence the slight uncertainty as to its elementary nature. Louyet obtained it by decomposing dry fluoride of silver by means of chlorine gas in vessels of fluor spar. He found the dry gas possessed affinities analogous to those of oxygen and sulphur; it acted upon almost all metals, but attacked glass feebly or not at all. Prat prepared it from fluoride of lead, and says that it decomposes water with intensity. Combined with hydrogen in the form of hydrofluoric acid, however, its most remarkable property is its rapidly corroding glass; and for this reason it is employed for etching.

Its presence is detected in any body that contains it, by submitting this in a vessel of platinum or lead, which are but slightly affected by the acid, to the action of concentrated sulphuric acid, and placing a plate of glass across the mouth of the vessel to receive the vapors evolved on the application of a gentle heat. This is the process by which hydrofluoric or fluohydric acid is obtained from fluor spar, the metallic vessel being a retort, furnished with a crooked neck of lead, in which the vapor condenses in the water placed in the bend to receive it, and which is kept cool by being surrounded with ice. It may also be obtained by condensing the vapors without the use of water in the lead tube; in this state it is called anhydrous fluohydric acid. The hy-drated acid is a colorless fluid, of specific gravity 1.06, boils at 80°, and cannot be made to congeal at any temperature. It has a strong affinity for water, its vapor rising and forming thick white fumes as it combines with the moisture in the air, until by dilution this action at last ceases. Dropped into water, a sound is produced with the fall of each drop, as if it had been red-hot iron. When diluted with water it is highly corrosive, and according to its strength may produce injury by touching the skin.

A single drop of the anhydrous acid may produce acute inflammation accompanied with fever. The marks made by the'gaseous acid when used for etching are fine and visible on account of their opacity, while those produced by the liquid are transparent, and must be deeply etched. The product of this action of the hydrofluoric acid upon silicious substances is the gaseous compound known as fluosilicic acid or fluoride of silicium; and thus is a means afforded of volatilizing silica and removing it from some of its combinations, by which their analysis is facilitated.