148 ORTIIOCLASE. 



until, with the aid of a gentle digestion, it is com- 

 pletely decomposed and dissolved, with exception of 

 some gelatinous silicic acid which is separated. The 

 whole solution is then evaporated to perfect dryness, 

 in order to render the silicic acid insoluble, the eva- 

 poration being conducted towards the last upon a 

 water-bath, with constant stirring. 



The saline mass is afterwards moistened with hydro- 

 chloric acid, a proper proportion of water added, and, 

 after digestion, the silica filtered off, washed, thoroughly 

 dried, ignited, and weighed in a covered crucible. 



From the solution the baryta is precipitated by 

 gradual and cautious addition of dilute sulphuric acid, 

 a great excess of which is to be carefully avoided ; the 

 sulphate of baryta is then filtered off and washed. (8ee 

 No. 3.) 



The filtrate is concentrated, if need be, by evapora- 

 tion, the alumina precipitated by sulphide of ammo- 

 nium, and treated as in No. 16. 



The liquid filtered from the alumina is evaporated 

 to dryness, and the dry mass ignited to expel the arn- 

 rnoniacal salts. This process requires so much the 

 more care, to avoid spirting, the more sulphate of am- 

 monia it contains, in consequence of the careless addi- 

 tion of sulphuric acid. 



At the end of the operation, in order to convert any 

 alkaline bisulphate into neutral sulphate, a fragment of 

 carbonate of ammonia is held in the covered crucible. 



The residue is sulphate of potassa, and is weighed 

 as such. Should soda also be present in the mineral, 

 the residue must be treated as in No. 4. 



Another method consists in precipitating most of 

 the baryta from the original solution by gradually 

 and cautiously adding dilute sulphuric acid; the rest 

 of the baryta, together with the alumina, is then pre- 

 cipitated by a mixture of carbonate of ammonia and 

 free ammonia, added in slight excess. 



