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SELECTED ARTICLES. 
the shooting of the crystals, and not by electricity of friction. 
If the hot solution of the transparent arsenious acid is allow- 
ed to cool rapidly, whereby a friable mass of arsenious acid is 
obtained; then either a very feeble light or none at all can be 
observed. Equally little light is observable if the transparent 
acid is treated with acetic or nitric acid, the latter either of the 
common strength or fuming. The reason of this is simply 
that these acids dissolve but very little of the arsenious acid, 
especially the acetic acid, so that this solution is but slightly 
tinged yellow by sulphuretted hydrogen, without any sulphu- 
ret of arsenic being precipitated. Dilute sulphuric acid, on 
the other hand, dissolves rather more arsenious acid by boil- 
ing, and if this solution be allowed to cool very slowly, a fee- 
ble light may sometimes be observed. If a large quantity of 
the transparent arsenious acid is treated with only so much 
nitro-muriatic acid (which, however, must contain an excess of 
muriatic acid) that it is not completely dissolved and oxidized 
to arsenic acid, a strong light is then observed on cooling. 
The cause of the luminosity of crystals during their forma- 
tion has long appeared to me to be this: that the substance 
which separates from a fluid in the form of a luminous crystal 
is not contained as such in the solution, but that it is only 
formed when the crystal is formed, and that the appearance of 
light is necessarily conditioned by the formation of a new sub- 
stance in a crystalline state. 
The light evolved during the crystallization of substances 
has most frequently been observed with sulphate of potash, 
but always only casually, and never during the recrystalliza- 
tion of pure sulphate of potash; but, as I believe, merely during 
the crystallization of the solution of the residue from the pre- 
paration of nitric acid. This contains almost always sesquisul- 
phate of potash, which as such is soluble in water, but which, 
according to Phillips, is decomposed whilst crystallizing into 
bisulphate and neutral sulphate of potash; and the latter be- 
comes luminous during crystallization, whilst it is formed in 
the fluid, and crystallizing out of it. 
Two isomeric states of the arsenious acid are commonly 
