GLASS. 209 



present may be detected by precipitating the solution 

 either with excess of ammonia or with carbonate of 

 lime, when those metals remain in solution and may 

 be precipitated by sulphide of ammonium. 



110. GLASS. 



Silicates of CaO, and KO or NaO, frequently also, of 

 PbO. 



Two analyses are made, one by fusion with an 

 alkaline carbonate, for the determination of silicic 

 acid ; the other by decomposing the glass with hydro- 

 fluoric acid, in order to estimate the alkali. 



I. The very finely-powdered glass fused with three 

 times its weight of carbonate of potassa and soda 

 (No. 10), the mass softened in water, dissolved in 

 dilute hydrochloric acid, evaporated to dryness, redis- 

 solved in water, acidulated with hydrochloric acid, 

 the silica filtered off and washed. 



From the solution, the small accidental impurities 

 of sesquioxide of iron, oxide of manganese, and 

 alumina, which are usually contained even in white 

 glass, are precipitated by ammonia, after the solution 

 has been mixed with some chlorine-water to perox- 

 idize the protoxide of manganese. 



The lime is afterwards precipitated by oxalic acid, 

 and the solution filtered from the oxalate of lime is 

 tested for magnesia, which may, moreover, have been 

 precipitated with the alumina. 



If the glass contain oxide of lead, that metal is pre- 

 cipitated by sulphuretted hydrogen from the solution 

 filtered from the silicic acid. 



II. For the determination of alkalies, a second quan- 

 tity of the very finely-powdered glass is decomposed 

 by hydrofluoric acid, or by ignition with carbonate of 

 baryta, as in the analysis of feldspar, the subsequent 



18* 



