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GEOLOGICAL MEMOIRS. 



of crystallization. This remarkable change also proves that nature 

 can accomplish in time what she cannot effect in a shorter space ; a 

 truth deserving to be remembered in all physical inquiries, and espe- 

 cially in geology. 



Some years after I received from the silver-mines of St. Andreas- 

 berg a piece of arsenikglas recently formed, which I placed in my 

 collection near the specimen before described. It has also now 

 acquired a porcellanous appearance, but has preserved a perfectly 

 smooth surface. I broke it across in order to ascertain the condition 

 of the interior. The inside is still perfectly glassy, the exterior only 

 being changed. It is also to be observed that the change from 

 without inwards has proceeded very differently in different parts. 

 On one part of the surface the thickness of the unchanged crust is 

 scarcely appreciable ; while in other parts the porcellanous mass (in 

 which the large-conchoidal fracture is changed into uneven, small-con- 

 choidal) is two lines thick, with its interior limit ill-defined. From 

 this it seems to follow, that in very similar masses of arsenikglas there 

 are certain differences of aggregation, which cause them to differ in 

 their progress towards becoming opake. On this it may depend, as 

 well as on other determining causes, that generally the amount of 

 change is independent of the length of time elapsed. For it is possible 

 that arsenikglas might be kept a longer time than the piece I have 

 described without exhibiting so remarkable a change. 



These observations on arsenious acid induced me to make some in- 

 quiries in order to understand better the relation of crystallized sul- 

 phuret of arsenic to the amorphous glass consisting of sulphur and 

 arsenic. The native realgar has the property of not melting to a glass, 

 but to crystallize in cooling. I exposed some compact realgar from 

 Tajowa in Hungary to a melting heat, over a spirit lamp, in an iron 

 spoon. It melted readily and contracted suddenly in cooling, and 

 formed small separate drusy masses studded with crystals. When 

 melted in a glass tube it forms deep concavities in cooling. The sur- 

 face immediately in contact with the glass has a fibrous structure, the 

 fibres being perpendicular to the exterior surface, while the inner 

 space is furnished with small crystals, in which the klino-rhombic 

 system is more or less clearly expressed. I kept a portion of realgar 

 in a closed tube at a melting heat for four hours, in order to see 

 whether the crystalline condition would not be changed into a vitreous 

 one by continued fusion ; but as before, small white crystals were 

 formed in cooling. The slower the cooling process was, the more 

 distinct and larger were the crystals ; but even a sudden cooling by 

 throwing the melted mass into water does not destroy the crystalline 

 action. These considerations caused Wohler to produce an artificial 

 realgar by melting together 1 equivalent of arsenic and 2 of sulphur, 

 which proved as crystalline as the masses produced by melting native 

 realgar. 



I received from Herr Seidensticker a compound of sulphur and 

 arsenic produced by sublimation, which in colour, the colour of its 

 powder, and in fracture resembles native realgar. In its small drusy 



