68 
THE GEOLOGIST. 
both appear non-crystalline, and both, therefore, mi,s[ht be confounded 
under the term " anaorphous ; " but the term " amorphous " is restricted to 
this particular form of solid silica having the low specific gravity. 
All attempts to crystallize silica by fusion have hitherto failed. Many 
experiments were made upon this subject a long time ago by means of the 
oxy-hydrogen blowpipe. Silica has been distinctly fused into small 
globules ; there is no great difficulty about that. More recently, Deville, 
w ho lias paid special attention to the application of high temperatures to 
metallurgical purposes, has succeeded in fusing silica in considerable 
quantities, and he has subjected it to slow cooling, but never in a single 
instance has there been the slightest trace of crystallization ; and such 
silica — silica fused at these high temperatures — has alwa3's the low specific 
gravity of 2'3. The bearing of this will be seen by-and-by upon the sup- 
posed formation of granite and certain other igneous rocks. If we take a 
piece of crystallized quartz of a high specific gravity, aud fuse it, we con- 
vert it into a substance somewhat resembling the silica of the low specific 
gravity, having the gravity of 2 3. Now that is apparently a small fact, 
but in a geological point of view it is one of the highest interest. Formerly, 
it was a marvel to melt a bit of platinum as big as a pin's head ; now, 
Deville has succeeded in melting it in mass. This platinum sometimes 
contains silicon, aud in fusion the silicon becomes oxidized, and converted 
into silica, — the melted sihca swimming on the top of the melted platinum 
in the form of a thin, transparent, colourless liquid. 
Gustave Eose found, that when perfectly transparent, entire rock- 
crystal underwent long exposure, say for eighteen hours, to a porcelain 
furnace, in which the temperature is exceedingly hi^h, — being estimated 
at about 2000° centigrade — there was no alteration in the specific gravity 
at this temperature ; but when the same crystal was exposed to the same 
conditions of temperature, having been previously pulverized — reduced to 
fine powder — its specific gravity was reduced from 2'6 to 2'3. Again, he 
found in the case of common flint having a specific gravity of 2 o91, owing 
to certain impurities Avhich interposed, that by exposure to this high tempe- 
rature for a long time, its specific gravity was reduced to 2*237 ; another 
example of the influence of high temperature in reducing the specific 
gravity of silica in this particular state of aggregation. 
We have now to consider more particularly this amorphous silica, or 
that form of silica to which Professor Graham has so well given the name 
of " colloidal, or jelly-like silica." This silica is obtained in various ways; 
one is by the decomposition of silicates by acids. Silicate of potash 
or silicate of soda will dissolve readily in water, the solution being known 
as water-glass ; if we add, under certain conditions, an acid to that water- 
glass, the silica will immediately separate in the form of jelly. Under other 
conditions of dilution there would be no immediate separation of jelly what- 
ever, but the -whole of the silica will be retained in solution, though it be 
separated from its combination with the base by the addition of a stronger 
acid ; but, on keeping, it will ultimately gelatinize, or during evaporation, 
by the application of heat, the silica will be thrown down in the jelly -like 
state. When evaporated to dryness, this jelly forms a white, amorphous 
powder. This gelatinized silica, or colloidal state of silica, is produced 
by the action of water on a peculiar compound termed " fluoride of sili- 
con,"' — a gaseous compound, consisting of silicon in combination with 
the element fluorine, ^^hich is an essential constituent of common fluor 
spar. It is a perfectly transparent, colourless gas, which immediately 
sufi'ers decomposition by contact with w atcr ; hence, when this gas 
is allowed to escape into the atmosphere, under ordinar}' circumstances 
