536 Intelligence and Miscellaneous Articles. 



casually, and never during the recrystallization of pure sulphate of 

 potash, but, as I believe, merely during the crystallization of the so- 

 lution of the residue from the preparation of nitric acid. This con- 

 tains almost always sesquisulphate of potash, which as such is solu- 

 ble in water, but which, according to Phillips, is decomposed whilst 

 crystallizing into bisulphate and neutral sulphate of potash ; and the 

 latter becomes luminous during crystallization, whilst it is formed in 

 the fluid, and crystallizing out of it. 



Two isomeric states of the arsenious acid are commonly known : 

 it is either transparent and vitreous, or porcellanous and opake. At 

 first after melting it is quite transparent, but simply by keeping it, 

 and without its experiencing any increase of weight, it becomes milk- 

 white and opake. In both states the acid has different specific gra- 

 vity and solubility in water. 



I have only been able to observe the evolution of strong light 

 during the crystallization of the arsenious acid, when I treated the 

 vitreous acid with muriatic acid in the above-mentioned manner. In 

 the same manner the opake acid and also the pulverulent arsenious 

 acid, which is obtained by sublimation during the roasting of the 

 arsenical ore, and which is known in commerce under the name of 

 " Giftmehl*," when treated with muriatic acid did not produce, even 

 by the most gradual cooling, any light, and it was only by shaking the 

 vessel that a very feeble light was visible ; in the latter case most 

 likely because the opake acid contained still some portions of the 

 vitreous acid. But this feeble light could never be compared with 

 the strong light which was visible when the transparent acid was 

 employed. The light evolved during the shooting of the crystals of 

 the arsenious acid appears, therefore, to depend upon this, that the 

 solution of the transparent acid is changed by crystallizing into the 

 opake or porcellaneous kind. The crystals produced belong, there- 

 fore, to the opake modification j and the change of the transparent 

 into the opake acid is caused by nothing else than the transformation 

 of the acid from a completely uncrystalline to a crystalline state. 



The crystals of arsenious acid which are obtained from a very 

 slowly cooled solution in muriatic acid are, however, transparent; 

 but this transparency is caused only by their size, and an aggregate 

 of very small crystals of the acid would exhibit an opake appearance. 

 The crystals formed were always regular octohedrons, and did not 

 possess the form observed by Wohler, which is, perhaps, a third iso« 

 meric modification of arsenious acid. 



If the transparent acid is treated in the above-mentioned manner 

 and proportions, and the crystals have been formed accompanied 

 by phosphorescence, and the whole been allowed to cool perfectly, 

 the phosphorescence can be obtained once more, and sometimes even 

 very powerfully, if the whole is again heated to the boiling-point and 

 slowly cooled. ' However, the light is much more feeble than that first 

 observed, and is only caused by the muriatic solution still containing 

 portions of the transparent arsenious acid, and which during crystal- 



• The suboxide of arsenic of Berzelius. 



