Thus by the action of hydrofluoric acid on manganese peroxide, or by decomposing a solution of hydrofluoric acid by an electric current, either oxygen or a mixture of oxygen and fluorine were obtained instead of fluorine. Probably a certain quantity of fluorine was set free by the action of oxygen or an electric current on incandescent and fused calcium fluoride, but at a high temperature fluorine acts even on platinum, and therefore it was not obtained. When chlorine acted on silver fluoride, AgF, in a vessel of natural fluor spar, CaF 2 , fluorine was also liberated; but it was mixed with chlorine, and it was impossible to study the properties of the resultant gas. Brauner (1881) also obtained fluorine by igniting cerium fluoride, 2CeF 4 = 2CeF 3 + F 2 ; but this, like all preceding efforts, only showed fluorine to be a gas which decomposes water, and is capable of acting in a number of instances like chlorine, but gave no possibility of testing its properties. It was evident that it was necessary to avoid as far as possible the presence of water and a rise of temperature; this Moissan succeeded in doing in 1886. He decomposed anhydrous hydrofluoric acid, liquefied at a temperature of -23° and contained in a U-shaped tube (to which a small quantity of potassium fluoride had been added to make it a better conductor), by the action of a powerful electric current (twenty Bunsen's elements in series). Hydrogen was then evolved at the negative pole, and fluorine appeared at the positive pole (of iridium platinum) as a pale green gas which decomposed water with the formation of ozone and hydrofluoric acid, and combined directly with silicon (forming silicon fluoride, SiF 4 ), boron (forming BF 3 ), sulphur, &c. Its density (H = 1) is 18, so that its molecule is F 2 . But the action of fluorine on metals at the ordinary temperature is comparatively feeble, because the metallic fluoride formed coats the remaining mass of the metals; it is, however, completely absorbed by iron. Hydrocarbons (such as naphtha), alcohol, &c., immediately absorb fluorine, with the formation of hydrofluoric acid. Fluorine when mixed with hydrogen can easily be made to explode violently, forming hydrofluoric acid.
In 1894, Brauner obtained fluorine directly by igniting the easily formed55 double lead salt HF,3KF,PbF4, which first, at 230°, decomposes with the evolution of HF, and then splits up forming 3KF,PbF2 and fluorine F2, which is recognised by the fact that it liberates iodine from KI and easily combines with silicon, forming SiF4.
This method gives chemically pure fluorine, and is based upon the breaking up of the higher compound—tetrafluoride of lead, PbF4, corresponding to PbO2, into free fluorine, F2, and the lower more stable form—bifluoride of lead, PbF2, which corresponds to PbO; that is, this method resembles the ordinary method of obtaining chlorine by means of MnO2, as MnCl4 here breaks up into MnCl2 and chlorine, just as PbF4 splits up into PbF2 and fluorine.
Among the compounds of fluorine, calcium fluoride, CaF2, is somewhat widely distributed in nature as fluor spar,56 whilst cryolite, or aluminium sodium fluoride, Na3AlF6, is found more rarely (in large masses in Greenland).
Cryolite, like fluor spar, is also insoluble in water, and gives hydrofluoric acid with sulphuric acid.
Small quantities of fluorine have also in a number of cases been found in the bodies of animals, in the blood, urine, and bones. If fluorides occur in the bodies of animals, they must have been introduced in food, and must occur in plants and in water. And as a matter of fact river, and especially sea, water always contains a certain, although small, quantity of fluorine compounds.