76 § 58. liASES AXD ACIDS ^¥1X11 11EAGE2^TS. 



from the filter as above, treat it with the alkaline solu- 

 tions also in the same manner, collect the residue that is 

 insoluble in the alkali on a dried and weighed filter, dry 

 and weigh it, and finally ignite and weigli again. The 

 first of the three weighings gives the total amount of 

 silica, sand, and coal, the second the sand and coal, and 

 the third the sand alone. 



h. If the silicate is insoluble in water or acids, pulverize 

 it until an impalpable powder is obtained, mix a weighed 

 quantity of it, in a platinum crucible, with four parts of 

 finely powdered potassic sodic carbonate, as intimately as 

 possible by stirring with a glass rod ; wipe the glass rod 

 with a little more of the carbonate on a slip of glazed 

 paper, and transfer this from the paper to the crucible ; 

 the latter should not, with all its contents, be more than 

 two-thirds filled. Cover it well and heat at first moder- 

 ately over a blast-lamp, or, after imbedding it in calcined 

 magnesia in a Hessian crucible, in a furnace ; carry the 

 heat gradually to an intense red ; after about 20 minutes 

 the mass will have ceased to boil and bubble, and the 

 operation is finished. Put the crucible, when cold, into a 

 beaker with" considerable water, and add hydrochloric 

 acid gradually, as directed for the solution of carbonates, 

 § 36 ; when the mass is entirely loosened from the cruci- 

 ble, take the latter out, rinse it carefully into the beaker, 

 transfer the contents of the beaker to a platinum or a 

 porcelain dish, evaporate to dryness, and eliminate silicic 

 acid, as in a. 



c. Of course potassium and sodium cannot be deter- 

 mined in the filtrate from the silica in 6, since both metals 

 have been added to the substance in a large and undeter- 

 mined quantity. 



For the determination of these elements the silicate 

 must be decomposed with the aid of hydrofluoric acid or 

 a fluoride. 



