Intelligence and Miscellaneous Articles, 535 



the crystallization of arsenious acid a strong emission of light, which 

 differs from that seen during the crystallization of other substances, 

 in as much as it may be produced at pleasure. Take two or three 

 drachms of the transparent or vitreous arsenious acid, put it in a 

 matrass of white glass along with an ounce and a half of not fuming 

 muriatic acid of the common strength, and half an ounce of water; 

 allow the whole to boil for ten minutes or a quarter of an hour, and 

 then let it cool as slowly as possible, which is best done by gradually 

 decreasing the flame of the spirit-lamp which has been used for the 

 boiling. If the experiment is conducted in a dark room, the crystal- 

 lization is accompanied by a strong emission of light, the formation 

 of each little crystal being attended by a spark. If the vessel is then 

 agitated, a great number of crystals suddenly shoot up, and an equal 

 number of sparks occur at the same time. If a considerable quantity 

 of arsenious acid, such as an ounce or an ounce and a half, or more, 

 is treated with a corresponding quantity of diluted muriatic acid, 

 then, on shaking the vessel, if the right moment is seized, the emis- 

 sion of light from the shooting of the crystals is so powerful that a 

 dark room may be lighted up by it. 



Considerable time elapses before the acid solution of arsenious acid 

 leaves off depositing crystals, consequently the cooled solution still 

 continues to emit light on the second and even on the third evening, 

 but only extremely feebly, and only when it is agitated. It is, how- 

 ever, impossible after this to produce any emission of light ; a proof 

 that it is occasioned by the shooting of the crystals, and not by elec- 

 tricity of friction. 



If the hot solution of the transparent arsenious acid is allowed 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 

 sulphuret of arsenic being precipitated. Dilute sulphuric acid, on 

 the other hand, dissolves rather more arsenious acid by boiling, and 

 if this solution be allowed to cool very slowly, a feeble light may 

 sometimes be observed. If a large quantity of the transparent arse- 

 nious acid is treated with only so much nitro-muriatic acid (which, 

 however, must contain an excess of muriatic acid,) that it is not com- 

 pletely dissolved and oxidized to arsenic acid, a strong light is then 

 observed on cooling. 



The cause of the luminosity of crystals during their formation 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 for- 

 mation of a new substance in a crystalline state. 



The light evolved during the crystallization of substances has most 

 frequently been observed with sulphate of potash, but always only 



