492 PRINCIPLES OF CHEMISTRY 



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

 vessels, because lead also acts on hydrofluoric acid, although only very 

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

 of lead is formed no further action takes place. Powdered fluor spar 

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

 even at the ordinary temperature, CaF 2 + H 2 S0 4 = CaSO 4 -f 2HF. At 

 130 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 

 specific gravity at 12-8 = O9849. 51 It dissolves in water with the evo- 

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

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

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

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

 tion distils over first. 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 attacks the skin, 



S1 According to Gore. 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. Its vapour density corresponds to the formula HP. 



6 * This composition corresponds to the crystallo-hydrate HCl^HjO. All the proper- 

 ties of hydrofluoric acid recall those of hydrochloric acid, and therefore the com- 

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

 acid at 85) must be explained by a polymerisation taking place at low temperatures, 

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

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

 observed. 



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

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

 describing that compound, forming gaseous silicon fluoride, SiO 2 + 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 cohesion is de- 

 stroyed. This is made use of in the arts, and in the laboratory, for etching designs and 

 scales, &c., 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 

 hydrofluoric acid, and it is soft enough to allow of designs being drawn upon it whose lines 

 lay bore the glass. The drawing is made with a steel point, and the glass is afterwards 

 laid in a lead trough in which a mixture of fluor spar and sulphuric acid is placed. The 

 sulphuric acid must be used in considerable excess, as otherwise transparent lines are 



