182 METALLOIDS. 



down upon it a cold white surface, like that of 

 a plate or saucer, the metal is again given up, 

 and reveals itself as a brownish black and high- 

 ly lustrous stain. The process may be con- 

 ducted in an ordinary vial, to which a pipe 

 stem, or glass tube has been fitted, by the 

 method before described. The above method 

 of detection is called Marsh's test. In a case of 

 suspected murder by poison, the moment of the in- 

 troduction of the pure porcelain into the flame, be- 

 comes one of the most intense interest. The gather- 

 ing stain, is at once the emblem of guilt and sentence 

 of ignominious death. 



446. EXPLANATION. The decomposi- 

 Sxplainthe t j on O f arsen i ous ac {d by hydrogen, in the 



above process. J J 



above experiment, and the reason of the 

 deposition of the metallic mirror, still remains to be ex- 

 plained. The nascent hydrogen affects the decompo- 

 sition of the acid, by a double action ; on the one hand 

 uniting with the metal to form arseniuretted hydrogen, 

 which escapes, and on the other hand, with its chlorine 

 to form hydrochloric acid, which remains behind. The 

 mirror of metal is deposited upon the plate or saucer, 

 because the introduction of the cold body into the 

 flame, so lowers its temperature that the metal itself can- 

 not burn. If the jet of gas is left to burn without in- 

 terference, both of its constituents are consumed to- 

 gether, and the flame assumes a blue color, from the 

 presence of the arsenic. 



