ORTHOCLASE. 149 



After twenty-four hours, the precipitate is filtered 

 off, washed, the alumina (together with the baryta) 

 extracted by dilute hydrochloric acid, precipitated by 

 freshly-prepared sulphide of ammonium, and rapidly 

 washed, with as little exposure to air as possible. 



The liquid containing the alkali, filtered off' from 

 the precipitate produced by carbonate of ammonia, is 

 concentrated by evaporation, acidified with hydro- 

 chloric acid, evaporated to dryness, and the saline mass 

 heated in a covered crucible ultimately to redness ; the 

 residue is chloride of potassium (or chloride of sodium). 

 It must be tested for baryta with sulphuric acid, since, 

 carbonate of baryta is not absolutely insoluble, and 

 would have been converted into chloride of barium by 

 ignition with salamrnoniac. 



II. A second method of decomposing feldspar is that 

 with hydrofluoric acid. The levigated mineral is placed 

 in a platinum dish, or in a capacious platinum crucible, 

 mixed with a suitable quantity of fuming hydrofluoric 

 acid, and digested with it to complete decomposition. 



Or the mineral may be spread out in a shallow plati- 

 num capsule, moistened with water, and exposed_for a 

 long time to the vapor of hydrofluoric acid, in an ap- 

 propriate leaden vessel closed with a lid. The hydro- 

 fluoric acid is evolved from powdered fluor-spar, which 

 is placed at the bottom of the vessel, moistened with 

 concentrated sulphuric acid, and gently heated. 



The mineral is however most easily decomposed by 

 fluoride of ammonium. One part of the mineral is 

 mixed with about six parts of the fluoride with a little 

 water, digested for some time and then raised to a low 

 red heat. 



When the decomposition of the feldspar is com- 

 pleted, the mass is gradually and cautiously mixed 

 with pure concentrated sulphuric acid, and evaporated, 

 slowly and carefully, to dryness. All the fluorine and 

 silicon are thus expelled, and after the volatilization ot 



13* 



