FLUORINE. 



By Prof. Henri Moissan, 

 Memhre de VAcademie des Sciences, Paris. 



There has long been known a curious mineral, fluor spar, which 

 occurs in nature in great cubic crystals, sometimes colorless, sometimes 

 tinted green or violet. This mineral is a binary compound of a metal, 

 calcium, united with another element hitherto impossible to isolate, 

 which has been named fluorine. 



This fluoride of calcium has very often been compared with the 

 chloride of sodium, the composition of which is perfectly well known 

 to chemists. In fact, there are great and profiouud analogies between 

 the fluorides and the chlorides; potassium chloride and potassium 

 fluoride both crystallize in tlie cubic system. In their chief properties 

 the chlorides resemble the fluorides. They usually give parallel reac- 

 tions; treated with sulphuric acid, both yield hydrogen acids which are 

 soluble in water and which fume strongly in the air. 



In addition to calcium fluoride, other compounds containing fluorine 

 are found in nature. We know, for example, a complex compound of 

 calcium phosphate and calcium fluoride which is called apatite. This 

 mineral, which occurs sometimes in very pretty crystals, has also been 

 obtained synthetically in the laboratory; but, which is more important, 

 Henri Sainte-Claire Deville has succeeded in preparing a chlorinated 

 apatite, and this new compound forms crystals identical wiih those of 

 the apatite containing fluorine. We may therefore say with propriety 

 that in these compounds chlorine can replace fluorine, or act as its 

 substitute. Here is a remarkable analogy, a bond wbich connects 

 well- studied, well-known chlorine with the elementary substance not 

 yet isolated, fluorine. 



Need I cite other examples'? They are not lacking. We know the 

 mineral wagnerite, which contains fluorine naturally, and we can pre- 

 pare the similar chlorinated compound. 



These analogies between chlorine and fluorine go still further. 



' A lecture delivered by Prof. Henri Moissan before the Royal Institution of Great 

 Britain, May 28, 1897. Translated from the French, as printed in Proceedings of the 

 Royal Institution, 1897. 



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