THE DETERMINATION OF SILICA. 
BY NICHOLAS KNIGHT. 
The method of procedure depends upon the amount in the sub- 
stance under investigation and also upon the condition of the 
silica, whether in the form of sand, or as a soluble or an insoluble 
silicate. The determination is beset with difficulties, but these are 
by no means insurmountable. It has been the object of much 
careful study and investigation and very accurate results are at- 
tainable even by persons who have not had much experience in 
quantitative analysis. 
There are two classes of minerals and rocks in which the analyst 
may be called upon to determine the amount of silica: In silicates 
proper, in which the percentage of silica is relatively large ; and in 
rocks and minerals like limestone, dolomite, phosphorite, and the 
various ores of iron and manganese, where the silica content may 
be quite small. 
In the important and growing Portland cement industry, there is 
often a call for the determination of the silica, as well as for com- 
plete analyses of the substances that enter into the cement. 
THE PREPARATION OF THE SPECIMEN. 
It is very desirable that the substance to be analyzed should be 
in a very finely divided state. It is usually reduced to a coarse 
powder with an ore crusher, or with a mortar and pestle of iron or 
steel. It is afterwards finely pulverized with an agate mortar and 
pestle. The whole must be thoroughly mixed before beginning the 
analysis to secure perfect homogeneity. The fine material should 
be sifted through a piece of fine, clean linen cloth, and the coarser 
particles should not be discarded, but should be rendered as fine 
as possible and added to the other portion. To keep only that 
which is more easily powdered, and to throw away the rest would 
result in not having a fair sample for the analysis. 
W. Hempel 1 has studied the abrading effect of pulverizing hard 
substances with mortars of iron, glass, agate, and hardened steel. 
He finds that for grinding hard substances like glass, hardened 
1. Zeits, Angew, Chemie 1901, page 843. 
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