THE HALOGENS 491 



formed 49bis double lead salt HF,3KF,PbF 4 , which first, at 230, 

 decomposes with the evolution of HF, and then splits up forming 

 3KF,PbF 2 and fluorine F 2 , which is recognised by the fact that it 

 liberates iodine from KI and easily combines with silicon, forming 

 SiF 4 . This method gives chemically pure fluorine, and is based upon 

 the breaking up of the higher compound tetrafluoride of lead, PbF 4 , 

 corresponding to PbO. 2 , into free fluorine, F 2 , and the lower more 

 stable form bifluoride of lead, PbF 2 , which corresponds to PbO ; that 

 is, this method resembles the ordinary method of obtaining chlorine 

 by means of MnO 2 , as MnCl 4 here breaks up into MnCl 2 and 

 chlorine, just as PbF 4 splits up into PbF 2 and fluorine. 



Among the compounds of fluorine, calcium fluoride, CaF 2 , is some- 

 what widely distributed in nature as fluor spar/' whilst cryolite, or 

 aluminium sodium fluoride, Na^AlF 6 , 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. 



Hydrofluoric acid, HF, cannot be obtained from fluor spar in glass 

 retorts, because glass is acted on by and destroys the acid. It is 



49 bu T. Nikolukin (1885) and subsequently Friedrich and Classen obtained PbCl 4 

 and a double ammonium salt of tetrachloride of lead (starting from the binoxide), 

 PbCl 4 2NH 4 Cl ; Hutchinson and Pallard obtained a similar salt of acetic acid (1893) 

 corresponding to PbX 4 by treating red lead with strong acetic acid ; the composition of 

 this salt is Pb(C 2 H 3 O._>) 4 ; it melts (and decomposes) at about 175. Brauner (1894) 

 obtained a salt corresponding to tetrafluoride of lead, PbF 4 , and the acid corresponding 

 to it, E^PbFg. For example, by treating potassium plumbate (Chapter XVIII. Note 55) 

 with strong HF, and also the above-mentioned tetra-acetate with a solution of KHFj, 

 Brauner obtained crystalline HK 3 PbF 8 i.e. the salt from which he obtained fluorine. 



90 It is called spar because it very frequently occurs as crystals of a clearly laminar 

 structure, and is therefore easily split up into pieces bounded by planes. It is called fluor 

 spar because when used as a flux it renders ores fusible, owing to its reacting with silica, 

 SiO 2 + 2CaF 2 = 2CaO + SiF 4 ; the silicon fluoride escapes as a gas and the lime combines 

 with a further quantity of silica, and gives a vitreous slag. Fluor spar occurs in mineral 

 veins and rocks, sometimes in considerable quantities. It always crystallises in the cubic 

 System, sometimes in very large semi-transparent cubic crystals, which are colourless or 

 of different colours. It is insoluble in water. It melts under the action of heat, and 

 crystallises on cooling. The specific gravity is 31. When steam is passed over incan- 

 descent fluor spar, lime and hydrofluoric acid are formed : CaF a + H 2 O = CaO + 2HF A 

 double decomposition is also easily produced by fusing fluor spar w^th sodium or potassium 

 hydroxides, or potash, or even with their carbonates ; the fluorine then pafesesover to the 

 potassium or sodium, and the oxygen to the calcium. In solutions for example, 

 Ca(NO 5 )2 + 2KF = GaF 2 (precipitate) -KSKNOg (in solution) the formation of calcium 

 fluoride takes place, owing to its very sparing solubility. 26,000 parts of water dissolve 

 one part of fluor spar. 



