hillebrand.] SILICA, SEPARATION FROM ALUMINA, ETC. 51 
in the open market or by special arrangeinenl with manufacturers, an 
article of sodium carbonate which can be called chemically pure 
With special precautions small lots can be prepared in the laboratory 
that will contain less than 1 milligram total impurity in 10 grams; but 
such an article can not be purchased in the market, and rarely will the 
so-called chemically pure dry sodium carbonate contain as little as 1 
milligram in 10 grams. The invariable contaminating substance-, aside 
from sand and straw, which have sometimes been found in large amount, 
are silica, alumina, iron, lime, and magnesia, all of these going into 
aqueous solution with the carbonate. The chief of these impurities 
are usually silica, alumina, and lime. An article of the above degree 
of purity is satisfactory in almost all imaginable cases, since the use 
of the usually extravagant amount of 10 grams for a fusion would 
introduce an error of but 0.1 per cent in the analysis, supposing 1 
gram of mineral to be operated on, and it would, moreover, be dis- 
tributed over several constituents. This error is undoubtedly fully 
equaled by the introduction of dust from the air in the various long 
evaporations. 
Precautions infusing. — Special directions with regard to the fusion 
and its first treatment are unnecessaiy, except to say that ordinarily 
from 4 to 6 parts of flux should be used to 1 of rock and that the flame 
should not be directed vertically against the bottom of the crucible, 
but at an angle against the side and bottom, nor should the flame be 
allowed to envelop the whole crucible. These precautions apply in 
all ignitions of reducible substances, and yet the} 7 are rarely observed. 
In neither case, if neglected, will there be the necessary oxidizing 
atmosphere within the crucible; on the contrary, reduction may occur 
fraught with serious consequences. This is especially true if the rock 
contains more than traces of pyrite or other sulphide, when, after 
cleansing and igniting the crucible, there ma} 7 appear on its interior a 
darkening due to oxidation of reduced iron which had alloyed with 
the platinum. This may in exceptional cases amount to several milli- 
grams in weight, and can be removed only by repeated ignitions, fol- 
lowed each time by scouring or treatment with hydrochloric acid or 
acid potassium sulphate. In order to avoid the use of niter in case of 
pyritiferous rocks, it is well to first roast the weighed powder in the 
crucible in which the fusion is to be made. 
Treatment after fusion. — When fusion is complete, the crucible La 
seized with the tongs (fig. 1, p. 23) and the contents are caused to 
solidify in a thin sheet over the sides and bottom by imparting an 
appropriate rotating motion with the arm during the cooling proc 
This is far preferable to allowing the melt to form a thick cake at 
the bottom, since much less time is required for disintegration, and 
separation from the crucible is usually much easier. 
It sometimes happens that the cooled flux, and even its solution. 
