202 IOWA ACADEMY OF SCIENCE 
steel is much superior to agate, and even green bottle glass is 
abraded less than agate. In finely pulverizing 10 grams of glass, 
an agate mortar and pestle 416 grams in weight lost 0.052 grams 
or 5.2 milligrams for each gram of the glass which was powdered. 
A hardened steel mortar and pestle of the same weight lost only 
one-tenth of that amount. It might often be desirable to grind one 
portion with steel apparatus for the silica determination and an- 
other portion with agate for the iron and other determinations. 
SILICA IN LIMESTONE AND DOLOMITE. 
These rocks for the most part are deep sea deposits and the 
amount of silica they contain is frequently very small. In some 
of the layers of the Niagara dolomites of Eastern Iowa, there is 
less than a half per cent of silica. The silica originated in the 
wearing away of older rock formations, igneous and metamorphic, 
as well as from non-metamorphic rocks, by the waves of the ancient 
seas, or it might have come from the marine fauna and flora. If 
from the older rocks, the silica can exist in a great variety of forms. 
It can be grains of pure sand, or small quartz crystals, or any of 
the numerous kinds of silicates. Ordinary acids would have little 
or no effect in decomposing most of the silicates. If the silica 
originated from non-metamorphic rocks, it is usually sand or clay, 
the hydrated aluminum silicate. 
For determining the silica in dolomites and limestones, fusion 
with sodium carbonate in a platinum crucible is recommended by 
some authorities, contrary to the practice of many chemists. 
This does not require as much alkali as does the decomposition of a 
silicate, % to V 2 a gram of alkali for each gram of substance being 
sufficient. 1 After the fusion the substance is transferred to a plat- 
inum or porcelain evaporating dish by the use of water and hydro- 
chloric acid. It is evaporated to dryness, stirring to a fine powder. 
Heating above the temperature of the steam bath accomplishes 
nothing except where much magnesium is present. Then 120° has 
been found to be the most favorable 2 temperature. Above 120°, the 
magnesium and silica will recombine, and the silica will again go 
into solution with hydrochloric acid. The dry and fine powder is 
moistened with concentrated hydrochloric acid, then with dilute 
hydrochloric acid and water, and after standing some time, the 
silica is filtered off. Two or three evaporations and treatment 
1. P. W. Shimer in Meade the Chemists Pocket Manual, page 162. 
2. J. P. Gilbert in Tech. Quarterly, Vol. 3, 1890, page 61. 
