IN OASES OF POISONING. 261 



4. A particle of the substance is placed in the end 

 of a very narrow tube (Fig. 40), and above it several 



Fig. 40. 



splinters of freshly-ignited charcoal so that they may 

 occupy about J inch of the tube. This part of the 

 tube is now held horizontally, in the flame of the spirit- 

 larnp, in such a manner that the spot where the arse- 

 nious acid is placed may remain without the flame. 

 When the charcoal is heated to redness, that portion 

 of the tube is also brought into the flame when the 

 volatilized arsenious acid, passing over the red-hot 

 charcoal, is reduced, and the metallic arsenic deposited 

 beyond the charcoal, in the form of a dark, lustrous, 

 metallic ring. By a gent]e heat, this metallic incrus- 

 tation may be carried still farther up the tube. If 

 the incrustation be chased hither and thither in the 

 tube it is oxidized, or at least partly, and converted 

 into small shining, colorless, volatile crystals of arse- 

 nious acid. If the tube be cut off, just before the part 

 which contains the metallic ring, and the latter then 

 gently heated, the characteristic garlic odor of arsenic 

 may be perceived on approaching the nose to the ori- 

 fice of the tube. 



5. This reduction of arsenic to the metallic state 

 may be effected with greater ease and certainty by 

 dissolving a small quantity of the substance in water 

 containing hydrochloric acid, and testing the solution 

 in Marsh's apparatus, in the manner to be presently 

 described more particularly. 



6. A particle of the arsenious acid is heated in a 

 small glass tube, closed at one end, with a piece of dry 



