486 PRINCIPLES OF CHEMISTRY 



and is distinguished by its poisonous properties, so that in working with 

 the acid a strong draught must be kept up, to prevent the possibility of 

 the fumes being inhaled. The non-metals do not act on hydrofluoric 

 acid, but all metals with the exception of mercury, silver, gold, and 

 platinum, and, to a certain degree, lead decompose it with the evolution 

 of gaseous hydrogen. With bases it directly gives metallic fluorides, 

 and behaves in many respects like hydrochloric acid. There are, how- 

 ever, several distinct individual differences, which are furthermore 

 much greater than those between hydrochloric, hydrobromic, and 

 hydriodic acids. Thus the silver compounds of the latter are in- 

 soluble in water, whilst silver fluoride is moderately soluble. Calcium 

 fluoride, on the contrary, is insoluble in water, whilst calcium chloride, 

 bromide, and iodide are not only soluble, but attract water with great 

 energy, owing to which gases are frequently dried by passing them 

 over calcium chloride. Neither hydrochloric, nor hydrobromic, nor 

 hydriodic acid acts on sand and glass, whilst hydrofluoric acid corrodes 

 them, forming gaseous silicon fluoride. The other halogen acids only 

 form normal salts, KC1, NaCl, with Na or K, whilst hydrofluoric acid 

 gives acid salts, for instance HKF 2 (and by dissolving KF in liquid 

 HF, KHF 2 2HF is obtained). This latter property is in close connec- 

 tion with the fact that at the ordinary temperature the vapour density 

 of hydrofluoric acid is nearly 20, which corresponds with a formula 

 H 2 F 2 , as Mallet (1881) showed ; but a depolymerisation occurs with a 

 rise of temperature, and the density approaches 10, which answers to 

 the formula HF. 54 



The analogy between chlorine and the other two halogens, bromine 



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

 lay bare 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 taken in considerable excess, as otherwise transparent lines 

 are obtained (owing to the formation of hydrofluosilicic acid). After being exposed 

 for some time, the varnish is removed (melted), and the design drawn by the steel 

 point is found reproduced in dull lines. The drawing may be also made by the direct 

 application of a mixture of a silicofluoride and sulphuric acid, which forms hydrofluoric 

 acid. In the laboratory a solution of hydrofluoric acid or its fumes is employed for 

 decomposing siliceous substances which are insoluble in ordinary acids, or else they are 

 fused with the salts KHP 2 or NH 4 F. 



54 Mallet (1881) determined the density at 30 and 100, previous to which Gore 

 (1869) had determined the vapour density at 100 3 , whilst Thorpe and Hambly (1888) 

 made fourteen determinations between 26 and 88, and showed that within this limit of 

 temperature the density gradually diminishes, just like the vapour of acetic acid. 

 The capacity of HF to polymerise into H 2 F 2 is probably connected with the property 

 of many fluorides of forming acids with HF for example, KHF 2 and H a SiF 6 . We 

 saw above that HC1 has the same property (forming, for instance, H a PtCl c , &c., p. 450), 

 and hence this property of hydrofluoric acid does not stand isolated from the properties 

 of the other halogens. 



