Atomic weight, 19. Fluorine is found in nature combined with calcium in the mineral fluorspar and is a constituent of cryolite a double fluoride ol sodium and aluminium found in Colorado and Greenlandand other minerals. It is also present in small quantities in bones and the enamel of teeth. Fluorine can be obtained in quantity by the electrolysis of fused potassium hydrogen fluoride or of anhydrous hydrofluoric acid at 230 C., and is, at ordinary temperatures, a pale yellowish gas of very active chemical, corrosive, and poisonous qualities, which attacks glass with avidity, bat is insoluble in water. When cooled to a temperature of187 degrees C. it condenses to the liquid state in the form of a mobile yellow fluid with a characteristic odour something like that of chlorine, and at a still lower temperature it assumes tiie solid form and is almost white. Fluorine acts strongly upon all metals, even gold and platinum to some extent, and many of them pass into a state of inflammation when thrown in a finely divided state into the gas. It also attacks organic substances with violence. A combination with hydrogen known as hydrofluoric acid gas is made by warming a mixture of strong sulphuric acid and powdered calcium fluoride in a leaden or platinum vessel: It is very soluble in water, and is largely employed for etching purposes, as, for example, making the graduations on glass measuring apparatus. The object to be marked in this way is coated with melted wax, and after making the design or marks on the coated glass, it is exposed to the action of the acid either in the form of gas or liquid, with the result that the glass is eaten into where exposed by the markings, the fluorine having no action on the wax. For some commercial purposes, an aqueous solution of this acid of 60 per cent, strength is produced. Hydrofluoric acid vapour is irritating and injurious to the respiratory organs, and the liquid produces ulcerated sores on the skin, and is altogether a very dangerous chemical compound. There are fluorides corresponding to the chlorides and also a number of double fluorides. The fluorides of potassium, sodium, and iron are but sparingly soluble in water, while the fluorides of silver and tin are easily soluble. Most of the fluorine compounds are easily fusible, and when ignited in a current of steam many of them are converted into oxides and hydrofluoric acid is evolved.
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