EVOLUTION OF LIGHT DURING CRYSTALLIZATION. 159 
known: it is either transparent and vitreous, or porcellaneous 
and opaque. 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 opaque. In 
both states the acid has different specific gravity 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 men- 
tioned manner. In the same manner the opaque acid and also 
the pulverulent arsenious acid, which is obtained by sublima- 
tion 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 opaque acid contained still some por- 
tions 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 opaque or 
porcellaneous kind. The crystals produced belong, therefore, 
to the opaque modification; and the chapge of the transparent 
into the opaque acid is caused by nothing else than the trans- 
formation 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 opaque appearance. The crystals formed were 
always regular octohedrons, and did not possess the form ob- 
* The suboxide of arsenic of Berzelius. 
