484 PKINCIPLES OF CHEMISTRY 



the action of fluorine on metals at the ordinary temperature is com- 

 paratively feeble, because the metallic fluoride formed coats the 

 remaining mass of the metals ; it is, however, completely absorbed by 

 iron. Hydrocarbons (for instance, naphtha), alcohol, c., immediately 

 absorb fluorine, with the formation of hydrofluoric acid. Fluorine when 

 mixed with hydrogen easily explodes violently, forming hydrofluoric 

 acid. 49 



Among the compounds of fluorine calcium fluoride, CaF 2 , is rather 

 widely spread in nature as fluor spar, 50 whilst cryolite, or aluminium 

 sodium fluoride, Na 3 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 by food, and must occur in plants and 

 water. And in reality river, and especially sea, water always contains 

 a certain, although small, quantity of fluorine compounds. 



49 According to Moissan, fluorine is disengaged by the action of an electric current 

 on fused hydrogen potassium fluoride, KHF 2 . The present state of chemical knowledge 

 is such that the notion of an element with its properties is much more general than the 

 notion of that element in the free state. It is profitable and agreeable to learn that even 

 fluorine in the free state has not succeeded in eluding experiment and research, that the 

 efforts to isolate it have been crowned with success, but the sum total of the general 

 chemical data concerning fluorine as an element gain but little by this result. The gain 

 will be augmented, however, if it be now possible to subject fluorine to a comparative 

 study in relation to oxygen and chlorine. There is particular interest in the phenomena 

 of the distribution of fluorine and oxygen, or fluorine and chlorine, competing under dif- 

 ferent conditions and relations. 



* 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 semitransparent 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 sp. gr. is 8'1. When steam is passed over incandescent 

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

 decomposition is also easily produced by fusing ftuor spar with sodium or potassium 

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

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

 Ca(NO 3 ) 2 + 2KF = CaF 2 (precipitate) 4-2KNO 3 (in solution) the formation of calcium 

 fluoride takes place, owing to its very sparing solubility. 26000 parts of water dissolve 

 one part of fluor spar. Fluorine in the form of calcium fluoride also enters into the 

 composition of certain minerals : but in general the amount of fluorine in minerals is 

 inconsiderable. Apatite is a mineral whose chief mass consists of calcium phosphate. 

 Apatites sometimes contain no fluorine whatever, but only chlorine ; whilst in other 

 instances a certain amount of fluorine enters into their composition, in which case the 

 atomic proportion of chlorine is proportionately diminished. 



