in Solid Arseniaus Acid and other Bodies. 347 



the director Herr Seidensticker (to whose management the 

 arsenic works there owe their excellence), a specimen of the 

 arsenic glass manufactured there, about two cubic inches in 

 size, which he had broken off with his own hands immediately 

 on opening the still warm apparatus, and had caused to be 

 instantly packed up, that it might arrive as little injured as 

 possible. The specimen, as I received it, had a distinct con- 

 choidal fracture, without a trace of crystallization ; it was 

 transparent and colourless, and altogether of a glassy appear- 

 ance. I laid it by in a drawer of my mineralogical collec- 

 tion, in a dry situation, close to my dwelling-room. A long 

 time passed before I had leisure to lay my hands on it again ; 

 when I did so some years after, its appearance was surpris- 

 ingly changed. Not only was the principal mass become 

 porcellaneous, but also on the opposite side the parts next 

 the surface had lost their clean conchoidal fracture, and to a 

 depth of two lines had adopted a circular structure, so that 

 the surface seemed rough and cracked. This change excited 

 my surprise, which was greatly increased when, at the end 

 of a few weeks, I found that not only had this acicular struc- 

 ture proceeded farther, and reached a depth of four French 

 lines in some places, but also that the exposed side of the 

 acicular masses was studded with a great number of dis- 

 tinct octohedral crystals (!) ; some of these crystals were half 

 a French line in diameter. They were collected in small 

 clusters, so as to give the whole surface a drusy, intumes- 

 cent appearance. The acicular-shaped parts of the crust 

 which were at right angles to the surface, passed insensibly 

 into crystals, the groups of which seemed, as it were, to be 

 pushed out beyond the surface. The crystals were white 

 like the rest of the mass, but more lustrous and more trans- 

 lucent." 



" Such a transformation of arsenic glass into a mass of 

 well-formed crystals, is a most remarkable instance of mole- 

 cular change in a rigid body, and is the more striking, since, 

 apparently, it is not caused by any exterior circumstance, nor 

 is attended by any change of constitution. It would seem 

 that the molecules are put in motion by a tendency of the 

 amorphous mass to pass from the condition of tension to that 



