and on the Origin of Granites. 30 
two distinct modifications of silicic acid in silicates. Whatever 
the value of this hypothesis may be, it results from the experi- 
ments of M. Rose, that, whatever may be the condition in which 
silica presents itself at the time of the decomposition of the sili- 
cates by acids, it possesses always the same properties. Its 
density is about 2°2, and rises to 2°3 by a somewhat prolonged 
ea)cination. 
The silica produced by the action of water on the gaseous 
fluoride of silicium, presents the same characters. 
The shells of infusoria are also formed of amorphous silica of a 
density of 2°2, rising to 2:3 by calcination in a porcelain kiln. 
The two modifications of silica are not merely distinguished 
by their density ; they differ very much in the resistance which 
they oppose to solvents. Solutions of caustic alkalies and alka- 
line carbonates, as well as hydrofluoric acid, dissolve the amorphous 
silica with very great facility when compared with crystalline 
silica ; this remark applies also to quartz rendered amorphous by 
fusion, as well as to amorphous silica obtained in the humid way. 
Amorphous silica is found in nature in the form of opal and 
hyalite. These minerals possess a density of about 2:14 to 
2:16. But these rather low numbers depend upon the fissured 
structure of the minerals ; for when their density is determined 
after having reduced them to a very fine powder, it is found to 
be about 2°2. 
Although opal is frequently found in plutonic rocks, as in 
basalts, it has probably not been formed there in a fused mass, 
but rather been produced by the prolonged action of water on 
the rocks. Besides, it is also found in rocks which certainly 
have not had an igneous origin—in the interior of fossils for 
instance. It is very often associated with the variety of silica 
whose density is 2°6. It has probably been formed by the soli- 
dification of a deposit of gelatinous silica, whilst the crystalline 
variety might have resulted from the slow deposit of silica in 
actual solution. . 
That which is sometimes found in the crevices of crucibles of 
blast furnaces, may be signalized as a particular variety of silica. 
M. Rose having examined some specimens, remarks that they 
all disengaged traces of ammonia when fused with caustic 
potash, whilst they disengaged nothing by simple calcination. 
This fact proves that this silica contains a little nitruret of silicium, 
and indicates that it must have been formed by the combustion 
of silica mixed with some of the nitruret. According to this it 
was very unlikely that it belonged to the crystalline variety. In 
fact M. Rose found its density only equal to 1:842 ; though it is 
true that this determination was made only wpon a very small 
mass. 
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