68 



THE GEOLOGIST. 



both appear non-crystalline, and both, therefore, might be confounded 

 under the term " amorphous ; " 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, Beville, 

 who has 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 always the low specific 

 gravity of 23. 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, and fuse it, we con- 

 vert it into a substance somewhat resembling the silica of the low specific 

 gravity, having the gravity of 23. 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, and in fusion the silicon becomes oxidized, and converted 

 into silica, — the melted silica swimming on the top of the melted platinum 

 in the form of a thin, transparent, colourless liquid. 



Gustave Hose 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 hifah, — 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 23. Again, he 

 found in the case of common flint having a specific gravity of 2 591, owing 

 to certain impurities which 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 waj^s; 

 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 diyness, 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, which is an essential constituent of common fluor 

 spar. It is a perfectly transparent, colourless gas, which immediately 

 suffers decomposition by contact with water ; hence, when this gas 

 is allowed to escape into the atmosphere, under ordinary circumstances 



