346 On Changes of Internal Structure observed 



On Interesting Changes of Internal Structure observed to take 

 place in Solid Arsenious Acid, and other Solid Bodies. 



The most remarkable property of arsenious acid, as Pro- 

 fessor Hausmann observes, is this,* that as an amorphous 

 body, without any admixture, and without losing its solid 

 state, it experiences a change which makes it assume a 

 totally different aspect. It has long been known, from expe- 

 riment, that the transparent arsenic glass gradually becomes 

 opaque, until it resembles porcelain. The substance, at first 

 colourless, becomes white, the transparency disappears, and 

 it becomes completely opaque ; the beautiful vitreous lustre 

 becomes feeble, and approaches the waxy. According to the 

 experiments of Taylor and of Guibourt, the specific gravity 

 at the same time diminishes. The former found that of the 

 transparent acid 3-798, and of the opaque 3-529. The latter 

 made the specific gravity of the transparent 3*7385, and the 

 opaque 3695. The hardness, also, is subject to change ; the 

 glass sometimes becoming pulverulent, so that its fracture is 

 earthy, and the lustre quite gone. 



Fuchs, in his beautiful work on Amorphism, has thrown 

 out the conjecture that the glassy arsenious acid loses its 

 transparency by virtue of a gradual change into a crystalline 

 mass. Again, in his Natural History of the Mineral King- 

 dom, he asserts it distinctly ; for he says " that the amorphous 

 arsenious acid in time becomes white, opaque, and porcel- 

 laneous, and also becomes pulverulent, so that it can scarcely 

 be recognised as crystalline. In order," says Professor 

 Hausmann, " to ascertain whether the crystalline struc- 

 ture could be detected in the altered arsenic glass, I 

 examined the crumbling outer crust under a magnifying 

 power of 400, but could not perceive any trace of it. 

 Though this experiment seems opposed to this view, yet 

 I have been lately satisfied of its truth in the most con- 

 vincing manner. In the year 1835, I received from the 

 silver furnaces of St Andreasberg, through the kindness of 



* Vide Journal of Geological Society, vol. vii. No. 25, p. 2, and Karsten and 

 Dechen. Archiv.fur Mineral, &c., 1850, vol. xxiii. p. 766. 



