86 University Geological Survey of Kansas. 
Silica and Insoluble Residue is the term applied to the above 
precipitate. After this is weighed, it may be tested for purity 
by heating with hydrofluoric acid. If any residue remains after 
this treatment, it should be tested for barium sulfate and other 
constituents by fusion with sodium carbonate. 
For Iron and Aluminum Ovxids, the filtrate from the silica and 
insoluble residue is treated by the ordinary methods, using 
ammonium chlorid and ammonium hydroxid. If the amount 
of this precipitate is considerable, the precipitate should be re- 
dissolved and again precipitated, using the same filter for col- 
lecting the precipitate the second time, and combining the 
filtrates. This precipitate, in addition to the above oxids, may 
contain phosphoric anhydrid, in case a qualitative examination 
has shown that to be present in the water. To determine the 
iron, after weighing, dissolve the precipitate in concentrated 
hydrochloric acid by digesting for some time, and determine 
the iron volumetrically in a sulfuric-acid solution by the ordi- 
nary methods. If phosphoric anhydrid was present, that should 
be determined in a separate sample of the water. The sum of 
this oxid and ferric oxid substracted from the total precipitate 
is calculated as alumina. 
For the determination of Calcium, the filtrate from the iron and 
aluminum hydroxids should be treated with ammonium oxalate, 
and the solution should be kept warm for some time. Incase much 
magnesium is supposed to be present, it is best to dissolve the 
precipitated calcium oxalate in hydrochloric acid and precipi- 
tate a second time, combining the filtrates. Calcium may be 
weighed as oxid or sulfate. 
As Magnesium still remains in the filtrate from the calcium, 
evaporate this filtrate to dryness with an excess of nitric acid in 
a porcelain evaporating dish, and finally heat over wire gauze. 
By this treatment, in ordinary waters the ammonium salts 
will nearly all be converted into ammonium nitrate, which, at 
a higher temperature, is broken up into nitrous oxid and water, 
both of which are readily volatile. In this way the excess of 
ammonium salts, which would interfere with the complete pre- 
cipitation of magnesium, is nearly all removed. The residue 
