﻿TRANSLATIONS AND NOTICES 



OF 



GEOLOGICAL MEMOIRS. 



On Arsenious Acid, Realgar, and Orpiment. 

 By Prof. Hausmann. 



[Karst. und Dech. Archiv fiir Mineral, u.s.w., 1850, vol. xxiii. p. 766.] 



Arsenious acid is known to occur in two forms, crystalline and 

 amorphous. These two isomeric states are distinguishable not only 

 by outward appearance and physical characters, but also by a different 

 chemical constitution, evidenced by their different solubility in water. 

 The amorphous arsenious acid when recent is a glassy substance, cha- 

 racterized by conchoidal fracture, vitreous lustre, and transparency : 

 the differences of hardness and specific gravity, usually observed 

 between the crystalline and amorphous varieties of a substance, are 

 perceptible in the two forms of arsenious acid. Karsten, who has 

 made such exact experiments on the specific gravity of so many simple 

 bodies and their combinations, has fixed the specific gravity of arse- 

 nious acid, procured by sublimation, at 3*7026, whilst he found that 

 of the acid, procured by digesting arsenic in nitric acid and washing it 

 out with water, to be 3*7202. The difference is still greater in hard- 

 ness ; since the hardness of the amorphous acid is equal to or even 

 surpasses that of calc spar, whilst that of the crystals is scarcely 

 greater than that of gypsum, and in some varieties is even interme- 

 diate between gypsum and common salt. 



Arsenious acid is sometimes found native. It has frequently been 

 confounded with pharmacolite, which it strongly resembles ; and 

 although in later mineralogical works it has been separated from that 

 substance under the name of Arsenik-bliithe, yet other mistakes have 

 crept in, from not distinguishing between the amorphous and cry- 

 stalline forms. To this is to be attributed the contradictory statements 

 of the hardness and specific gravity which occur in the latest works 

 on mineralogy. Kobell, in his 'Principles of Mineralogy ' in 1838, 

 puts the hardness of arsenious acid at 3*5, which accords with the 

 greatest hardness of the amorphous acid. The newest edition of the 

 physiographic division of Mohs's ' Principles of the Natural History 

 of the Mineral Kingdom,' by Zippe in 1839, gives the specific gravity 

 3*698, as determined by Roger and Dumas, which refers to the trans- 

 parent arsenikglas ; whilst the hardness, which is called 1*5, belongs 

 to the softer varieties of the crystallized acid. In some works the 

 numbers are in excess. Breithaupt in his ' Hand-book of Mineralogy' 



VOL. VII. PART II. B 



