252 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 

 formed according to the equation : 



(Hydrosilico- 

 fluoric acid.) 



Davy attempted to isolate fluorine by passing an electric 

 current through hydrofluoric acid. This method was used 

 successfully by Moissan in 1886 (Ann. Chim. Phys. 1887, 

 [vi], 12, 473). An electric current was passed through a 



FIG. 42. MOISSAN'S APPARATUS FOR PREPARING FLUORINE. 



mixture of hydrogen fluoride and potassium fluoride, con- 

 tained in a platinum U-tube (Fig. 42), and cooled to 

 -23 C. by the evaporation of methyl chloride. The elec- 

 trodes, //, of platinum iridium alloy, were insulated from the 

 U-tube by stoppers, FF, of fluorspar, the joints being made 



