348 Changes on Arsenious Acid and other Bodies, 



of repose and equilibrium, which is the characteristic condi- 

 tion of crystallization. This remarkable change also proves 

 that nature can accomplish in time, what she cannot eiFect in 

 a shorter space ; a truth deserving to be remembered in all 

 physical inquiries, and especially in geology." 



" Some years after, I received from the silver mines of 

 St Andreasberg a piece of arsenic glass recently formed, 

 which I placed in my collection, near the specimen before 

 described. It has also now acquired a porcellaneous appear- 

 ance ; but has preserved a perfectly smooth surface. I broke 

 it across, in order to ascertain the condition of the inte- 

 rior. The inside is still perfectly glassy, the exterior only be- 

 ing 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 un- 

 changed crust is scarcely appreciable ; while, in other parts, 

 the porcellaneous mass (in which the large conchoidal frac- 

 ture is changed into uneven, small conchoidal) is two lines 

 thick, with its interior limit ill defined. From this it seems 

 to follow, that in very similar masses of arsenic glass there are 

 certain differences of aggregation, which cause them to differ 

 in their progress towards becoming opaque. On this it may 

 depend, as well as on other determining causes, that gene- 

 rally the amount of change is independent of the length of 

 time elapsed. For it is possible that arsenic glass might be 

 kept a longer time than the piece I have described without 

 exhibiting so remarkable a change." 



Similar changes of structure are observed to take place in 

 calcareous minerals, and probably changes of the same de- 

 scription occur in many other minerals, and also in rocks — 

 among artificial substances exhibiting such changes we may 

 particularise the confection called Barley-sugar. This sub- 

 stance, when newly prepared, has a pale wine-yellow colour 

 and is more or less translucent. Its fracture is conchoidal, 

 with a shining vitreo-resinous lustre. If kept for a time, its 

 translucency diminishes, and gradually the conchoidal fracture 

 disappears, and in its place the whole mass of the substance 

 assumes a beautiful stellular radiated structure. Geologically 

 considered, this topic is of great importance. — Editor FhiL Jour. 



