37 



Wash the residue, by decantatiou, several times with hot water and 

 then transfer it to the filter, and to make sure of removing every trace of 

 the sodium carbonate which may still adhere to it, with water slightly 

 acidulated with hydrochloric acid, and finally with hot water. 



This will dissolve the silica in combination with the bases of the clay, 

 and also the hydrated silica, and leave a residue of quartz sand and 

 silicates in the form of sand, e. </., feldspar sand, which is dried, ignited, 

 and weighed: > 



The difference between this last weight a.nd that of the insoluble resi- 

 due A will give the amount of hydrated silica. Or this may be deter- 

 mined directly by acidifying the filtrate with hydrochloric acid, evap- 

 orating to dryness, and driving off the chlorine, taking up with dilute 

 hydrochloric acid, filtering, washing well, drying, igniting, and weigh- 

 ing the hydrated silica. (Fres., 209.)- 



DETERMINATION OF THE SOLUBLE SILICA. 



Evaporate the main solution A to dryness in .a porcelain dish, adding 

 a little, about 2 c. c., of nitric acid to destroy the organic matter and 

 oxidize the iron. Then heat in the a'ir-bath at 110 C. until the acid 

 fumes disappear. Bj holding a glass rod moistened with ammonia in 

 the dish and noting when the white fumes cease to be produced, an 

 easy means is afforded of telling when the hydrochloric acid is entirely 

 driven off. 



After the mass is thoroughly dried moisten it with 25 c. c. dilute 

 hydrochloric acid, heat to a temperature just below boiling for twenty 

 or thirty minutes, and dilute with 50 c. c. of hot water ; everything 

 should be in solution except the silica. Filter this out, and wash with 

 hot water until the washings give no reaction for chlorine when tested 

 with argentic nitrate. Dry the precipitate on the filter at 110 0., and 

 transfer it to a weighed platinum crucible, and heat at a low temper- 

 ature until the paper is consumed. Care must be taken that the heat is 

 not too strong at first, as there is great danger of some of the silica pass- 

 ing off with the volatile matter. Gradually raise the heat until the 

 silica becomes white. Then cool the crucible and contents in a desic- 

 cator, weigh, and calculate the per cent, of soluble silica. (Fres., 140, 

 II, a.) 



DETERMINATION OF THE ~ IRON AND ALUMINA. 



Make the filtrate from the soluble silica up to 500 c. c., solution B. 



To 200 c. c. of the solution, equal to 4 grams of the air-dried soil, add 

 ammonia to alkaline reaction, to precipitate the aluminium and ferric 

 hydrates, together with the phosphoric acid. Be careful not to add a 

 large excess, or time will be wasted in boiling it out, which will be 

 necessary, for the reason that alumiuiuni hydrate is somewhat soluble 

 in excess of ammonia. Boil the solution until the vapors no longer 

 smell of ammonia, and do not turu turmeric paper brown. Allow the 



