METHODS OF CLAY ANALYSES. 47 



The silica is washed thoroughly with boiling water 

 and then ignited in a platinum crucible, weighed, and 

 moistened with concentrated sulphuric acid. Hydro- 

 flouoric acid is cautiously added until all the silica has 

 disappeared. The solution is evaporated to dryness 

 upon a sand-bath, ignited and weighed. The differ- 

 ence in weight is silica. 



Iron Sesquioooide — The filtrate from the silica is 

 divided into equal portions. To one portion in a reduc- 

 ing flask is added metallic zinc and sulphuric acid. 

 After reduction and filtration to free the liquid from 

 undissolved zinc and carbon, the iron is determined 

 by titration with a standard solution of potassium 

 permanganate. 



Aliwiinium Owide-^-^To the second portion, which 

 must be brought to boiling, ammonium hydroxide is 

 added in slight excess, the boiling continued from two 

 to five minuts, the precipitate allowed to settle and 

 then caught-upon the filter, all of the chlorides being 

 washed out with boiling water. The precipitate is 

 ignited and weighted as a mixture of aluminium oxide 

 and iron sesquioxide. The amount of iron sesquioxide 

 already found is taken from this and the remainder 

 reported as alumina. 



"Calcium Oxide — The filtrate from the precipitate 

 of iron and aluminium hydroxides is concentrated to 

 about two hundred cubic centimetres, and the calcium 

 precipitated in a hot solution by adding one gram of 

 ammonium oxalate. The precipitate is allowed to 

 settle during twelve hours, filtered and washed with 

 hot water, ignited and weighed as calcium oxide. 

 When the calcium is present in notable amounts, the 

 oxide is converted into the sulphate and weighed as 

 such. 



Magnesium Oxide — The filtrate f'^om the calcium 



