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GEOLOGICAL MEMOIRS. 



for 1841, puts the hardness at from 3 to 4, which agrees with 2*5 — 3 

 of Mohs, and which exceeds not only the hardest varieties of the cry- 

 stallized but also of the amorphous acid. The specific gravity given 

 in the same work belongs only to the latter. Fuchs in his * Natural 

 History of the Mineral Kingdom' for 1842, compares the hardness 

 of arsenious acid to that of calc-spar, which is true of the arsenikglas, 

 but is never reached by the crystallized acid. Haidinger in his ' Hand- 

 book of Mineralogy' for 1845, gives 1*5 for the hardness, and 

 3" 6 — 3*8 as the limits of the specific gravity, within which both va- 

 rieties fall. I have to acknowledge that in the new edition of my 

 ' Mineralogy,' in describing arsenik-bliithe I neglected to notice this 

 distinction between the amorphous and the crystalline forms. 



Most of the varieties of arsenik-bliithe, which in nature is usually 

 found as a secondary product in veins containing arsenic and arsenical 

 salts, belong to the crystallized form ; for although perfect and pure 

 crystals are very seldom met with, yet the crystalline form may be 

 more or less distinctly observed in the foliated, acicular, or capillary 

 structure. Only the pulverulent variety, found in the Katharina 

 Neufang Mine at St. Andreasberg, and possibly some stalactitic va- 

 rieties found elsewhere, can be looked on as amorphous. This variety 

 under the name of arsenikglas must be separated in our system from 

 arsenik-bliithe as a distinct species ; and upon the same grounds on 

 which opal is separated from quartz. Greater hardness and increased 

 solubility in water give arsenikglas distinct characters, independent of 

 the want of crystalline form. 



Besides the products containing arsenious acid which are made 

 designedly by metallurgic processes, this substance is sometimes ac- 

 cidentally formed in the furnaces, both amorphous and crystalline. 

 More or less perfect crystals are formed not unfrequently in roasting 

 arsenical ores, and as products of the furnace, as is the case at the 

 furnaces at Ocker, near Goslar, and at St. Andreasberg. Crystals are 

 sometimes formed in the walls of the furnaces in which arsenical ores 

 have been smelted, or such as accidentally contain arsenic. I possess 

 specimens from the silver furnaces of St. Andreasberg and from the 

 copper furnaces of Reichelsdorf, in which latter the arsenious acid is 

 found in the upper part of the c schiefer-ofen' even in the fibrous 

 state. In roasting the furnace-products containing arsenic, some- 

 times arsenikglas is obtained in the form of incrustations and stalac- 

 tites. 



The most remarkable property of arsenious acid incontestably 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 experiment 

 that the transparent arsenikglas gradually becomes opake, until it re- 

 sembles porcelain. The substance at first colourless becomes white, 

 the transparency disappears, and it becomes completely opake ; the 

 beautiful vitreous lustre becomes feebler 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 3798, and of the opake 3*529. The latter made 



