BROMINE IODINE-FLUORINE. 127 



minerals, in some waters, and also in the enamel of teeth, and in the 

 bones of mammals. Fluorine was, until 1887, scarcely known in the 

 elementary state, because all attempts to isolate it were frustrated by 

 the powerful affinities which this element possesses, and which render 

 it difficult to obtain any material (from which a vessel may be made) 

 which is not chemically acted upon, and, therefore, destroyed, by 

 fluorine. The method used now for liberating fluorine depends upon 

 the decomposition of hydrofluoric acid by a strong current of electricity 

 in an apparatus constructed of platinum with stoppers of fluorspar. 

 To prevent too rapid corrosion of the platinum vessels, the decom- 

 position is accomplished at a temperature below the freezing-point. 

 Fluorine is a gas of yellowish color, having a highly irritating and 

 suffocating. odor, and possessing affinities stronger than those of any 

 other element. As a supporter of cojabustion, fluorine leaves oxygen 

 far behind ; it combines sp&j^ajgteously even in the dark and at low 

 temperature with hydrogen ; sulphur, phosphorus, lampblack, and 

 also many metals ignite readily in fluorine ; even the noble metals, 

 gold, platinum, and mercury, are converted into fluorides; from 

 sodium chloride the chlorine is liberated with the formation of 

 sodium fluoride ; organic substances, such as oil of turpentine, alco- 

 hol, ether, and even cork ignite spontaneously when brought in 

 contact with this remarkable element. 



Hydrofluoric acid, HP (Hydrogen fluoride). A colorless gas, very 

 irritating, soluble in water. It is obtained by the action of sulphuric 

 acid on fluorspar : 



CaF 2 -f H 2 SO 4 = 2HF -f CaSO 4 . 



Hydrofluoric acid, either in the gaseous state or its solution in 

 water, is used for etching on glass. This effect is due to the action of 

 the acid upon the silica of the glass, which is converted into either 

 silicon fluoride, SiF 4 ; or into hydrofluosilicic acid, H 2 SiF 6 . 



Hydrofluoric acid, or strong solutions of it, are powerful antiseptics. In 

 small quantities the acid is used as an admixture to fermenting liquids, as it 

 has been found that it does not act upon the principal ferment of yeast, which 

 causes the decomposition of sugar into alcohol and carbon dioxide, while it 

 readily destroys a number of objectionable ferments. The yield of alcohol is 

 thus considerably increased. 



Experiment 15. Prepare a glass plate by heating it slightly and covering its 

 surface with a thin layer of wax or paraffin ; after cooling, scratch some letters 

 or figures through the wax, thus exposing the glass. Set the plate over a dish 

 (one made of lead or platinum answers best), in which a few grammes of pow- 

 dered fluorspar have been mixed with about an equal weight of sulphuric acid, 



