68 
THE GEOLOGIST. 
from its aspect, its crystalline form,* and its other properties, it was 
impossible to distinguish from the Cornwall specimens. 
The masonry near the springs was often seen to be impregnated with 
hyalite (a sort of transparent silica) in every respect similar to the 
hyalite found in basalt rocks ; and sometimes apophyllite (silicate of 
potash and lime) showed itself in very neat crystals. We would 
observe here Wochler succeeded formerly in dissolving and crystallizing 
this mineral by the aid of water at 180" Centigrade. 
M. Daubree asked himself this question : If we find hydrated silicates 
formed slowly in mineral springs at no very high temperature, is it not 
probable that anhydrous silicates may be artificially and more quickly 
produced by the action of water at a higher temperature ? Numerous 
experiments have answered this question in a most affirmative manner. 
They consisted principally in submitting the different substances in 
presence of water, to a heat of 400" Centigrade for a month together, in 
a closed glass tube, enveloped and protected by an iron case. 
As glass formed part of the apparatus, it naturally occurred to him to 
determine first of all, what would be the result of this treatment on 
glass itself. It was soon found that this substance undergoes, by the 
simple action of water at this temperature, a complete decomposition : 
it first becomes opaque, earthy, and fragile, resembling kaolin, then it 
gradually and regularly swells and transforms itself into a host of 
minute crystals, which were found on examination to be woUastonite 
(8 Ca 0, Si 0^) ; at the same time the alcalis of the glass, and a certain 
quantity of silica, are dissolved. Soon, however, silica is deposited in the 
form of crystallised quartz, of which some of the crystals measure two 
millimeters in length. The presence of alumina modifies these phenomena; 
thus, when obsidian was acted upon in like manner, minute crystals of 
feldspar were obtained, resembling, en viasse, granular trachyte. Clay 
and kaolin, which had been previously purified by washing, on being sub- 
mitted to a similar treatment, gave birth to feldspar, mixed with 
crystals of quartz ; and if the glass, in the first experiment, is decom- 
posed in presence of oxide of iron, not wollastonite, but pyroxen, is 
obtained — the samples of which immediately remind us of the natural 
crystals found in the Tyrol, Piedmont, at the Somrna, &c. They are 
••■ The natural specimens of sulpliiJe of copper arc cHmnrphous with tiie 
sulphide of the laboratory. 
