THE HALOGENS 485 



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

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

 It is prepared in lead vessels, and when it is required pure, in platinum 

 vessels, because lead in reality also acts on hydrofluoric acid, only very 

 feebly at the surface, and when once a coating of fluoride and sulphate 

 of lead is formed there is no further action. Powdered fluor spar 

 and sulphuric acid evolve hydrofluoric acid (which fumes in the air) 

 even at the ordinary temperature, CaF 2 + H 2 SO 4 = CaSO 4 + 2HF. At 

 1 30 fluor spar is completely decomposed by sulphuric acid. The acid is 

 then evolved as vapour, which may be condensed by a freezing mixture 

 into an anhydrous acid. The condensation is aided by pouring water 

 into the receiver of the condenser, as the acid is easily soluble in cold 

 water. 



In the liquid anhydrous form hydrofluoric acid boils at +19, and 

 its sp. gr. at 12 > 8 = 0'9849. 51 It dissolves in water with the evolution 

 of a considerable amount of heat, and gives a solution of constant 

 boiling point which distils over at 120 ; hence the acid is able to 

 combine with water. The specific gravity of the compound is 1*15, and 

 its composition HF,2H.,O. 52 With an excess of water a dilute solu- 

 tion first distils over. The aqueous solution and the acid itself must 

 be kept in platinum vessels, but the dilute acid may be conveniently 

 preserved in vessels made of various organic materials, such as gutta- 

 percha, or even in glass vessels having an interior coating of paraffin. 

 Hydrofluoric acid does not act on hydrocarbons and many other sub- 

 stances, but it acts in a highly corrosive manner on metals, glass, porce- 

 lain, and the majority of rock substances/ 53 It also corrodes the skin, 



51 According to Gore, hydrofluoric acid remains liquid when cooled to 34. Fremy 

 obtained anhydrous hydrofluoric acid by decomposing lead fluoride at a red heat, by 

 hydrogen, or by heating the double salt HKF. 2 , which easily crystallises (in cubes) from 

 a solution of hydrofluoric acid, half of which has been saturated with potassium hydroxide, 

 or carbonate. 



5 - This "composition corresponds with the crystallo-hydrate HC1,2H. 2 O. All the 

 properties of hydrofluoric acid call to mind those of hydrochloric acid, and therefore the 

 comparative ease with which hydrofluoric acid is liquefied (it boils at +19, hydrochloric 

 acid at 35), must be explained by a polymerisation taking place at low temperatures, 

 as will be afterwards explained, H.>F 2 , being formed, and therefore in a liquid state it 

 differs from hydrochloric acid, for which a phenomenon of a similar kind has not yet been 

 observed. 



53 The corrosive action of hydrofluoric acid on glass and similar siliceous compounds 

 is based upon the fact that it acts on silica, SiO.i, as we shall consider more fully in 

 describing that compound, forming gaseous silicon fluoride, SiO.> + 4HF = SiF 4 + 2H. 2 O. 

 Silica, on the other hand, forms the binding (acid) element of glass and of the mass of 

 mineral substances forming the salts of silica. When it is removed the bond is 

 destroyed. This is made use of in practice, and the laboratory, for etching designs and 

 scales, <fec., on glass. In engraving on glass the surface is covered with a varnish com- 

 posed of four parts of wax and one part of turpentine. This varnish is not acted on by 



