234 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



produced with much heat, and an acid fluid not disagreeable 

 to the taste is formed if the water be in sufficient quantity. 

 It instantly corrodes and dissolves glass " (Works, IV. 

 347348). 



Hydrofluoric acid. The acid vapour prepared by the 

 action of oil of vitriol on fluor spar had been used since 1670 

 at Nuremberg for etching glass. The liquid acid was dis- 

 covered in 1771 by Scheele (Essays, pp. 1-51), who called it 

 " fluor acid " ; but it was first obtained in its pure form in 1809 



FIG. 41. TWINNED CUBES OF FLUOR SPAR. 

 British Museum (Natural History). 



by Gay-Lussac and Thenard, who distilled the acid in metal 

 vessels in order to avoid contamination by glass. Davy at 

 first regarded the liquid acid as a compound of water with an 

 anhydrous oxy-acid, strictly analogous with sulphuric acid. 

 But, after recognising chlorine as an element, he "was 

 forcibly struck by the analogy" between the chlorine 

 compounds and those prepared from fluor spar. After 

 many fruitless attempts to isolate oxygen from them, Davy 

 concluded : 



