142 NATRGLITE. 



insoluble; the evaporation is best effected, at least 

 towards the end, in th'e water-bath. 



The remaining salt mass is moistened with hydro- 

 chloric acid, after some time a little hot water is poured 

 upon it, and the silica filtered off, washed with hot 

 water, dried, carefully ignited and weighed. 



From the filtered solution the alumina is precipitated 

 by sulphide of ammonium, and treated as in No. 16. 



When the zeolite contains sesquioxide of iron, the 

 alumina is colored more or less black or greenish- 

 black by sulphide of iron. The iron is separated from 

 it as in No. 21. 



The solution filtered from the alumina is evaporated 

 to a small bulk, in a dish, transferred to a weighed 

 platinum crucible, and carefully evaporated to drjmess. 

 The saline mass is gradually heated till the sal-ammo- 

 niac is volatilized, and finally heated to redness, the 

 cover being loosely placed upon the mouth of the cru- 

 cible. The residue is chloride of sodium. 



The silicic acid obtained in the decomposition of a 

 silicate, especially if it separate rather in a pulverulent 

 than a gelatinous state, must always be examined as 

 to its purity, since it may sometimes contain other con- 

 stituents of the mineral, especially alumina, or even 

 portions of the undecornposed mineral itself. 



Pure silicic acid must entirely dissolve in a boiling 

 solution of carbonate of soda. An insoluble residue 

 indicates an impurity, which should be filtered off arid 

 examined. It is preferable, however, to fuse such 

 silicic acid with 3 parts of carbonate of potassa and 

 soda, and to treat it as in No. 79 or 80. 



Silicic acid, if perfectly pure, when dissolved in 

 hydrofluoric acid, in a platinum capsule, entirely dis- 

 appears on evaporation. Should there be any residue, 

 it is again treated with hydrofluoric acid, afterwards 

 with concentrated sulphuric acid, evaporated to dry- 



