xii THE HALOGENS 251 



of carbon is called CARBONIC ANHYDRIDE instead of carbonic 

 acid, whilst the name CARBONIC ACID is applied to an 

 unstable hydrate, hydrogen carbonate, which is produced 

 when the gas is dissolved in water. 



SUMMARY AND SUPPLEMENT 

 A. FLUORINE 



Davyin 1813-14 recognised the presence in fluorspar (calcium 

 fluoride, CaF 2 ) and in hydrofluoric acid (hydrogen fluoride, 

 HF ; described by Scheele in 1771 as "fluor acid") of an 

 element fluorine, closely resembling chlorine. The analogy 

 between hydrofluoric acid and hydrochloric acid appears in its 

 physical properties, in its method of preparation : 



CaS0 4 + 2HF. 



(Fluor- (Hydrofluoric 



spar.) acid.) 



2NaCl + H 2 SO 4 -> Na 2 SO 4 + 2HCl. 



(Rock-salt.) (Hydrochloric 



acid.) 



and in the action of metals upon it : 



+H 2 , 

 2HCT+2K->2KCl+H t 5 



but mainly in the fact that no oxygen can be extracted from 

 any of the fluoric compounds. The acid acts readily on the 

 silica present in glass : 



4HF + SiO 2 ^ SiF 4 + 2H 2 O, 



(Silica.) (Silicon 



tetrafluoride.) 



giving rise to a gas, silicon tetrafluoride, SiF 4 , which is decom- 

 posed again by water, somewhat in the manner indicated by the 

 lower arrow in the above equation ; the actual product of the 

 decomposition by water is, however, a hydrosilicofluoric acid : 



(SiF 4 -f2HF = H 2 SiF 6 ) 



