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 

 Bimple 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 wollastonite 

 (3 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 

 f(4dspar were obtained, resembling, en masse, 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 Somma, &c. They are 



Tlic uiitiival ^pochueus of sulphide of copper are dimorphous with tlie 

 sulpliiilc of (ho luboratory. 



