MICROCHEMICAL REACTIONS OF ARSENIC 397 



stibine is evolved, only arsine passes off with the hydrogen. 

 Metallic antimony is precipitated in part and deposited in part 

 upon the aluminum. 



In place of a crystal fragment of silver nitrate we may employ 

 a fragment of mercuric bromide or a textile fiber soaked in mer- 

 curic bromide and dried; in the latter case a much finer capillary 

 tube can be used and the delicacy of the reaction is somewhat 

 increased. Arsine turns mercuric bromide red or brown. 



B. By Reduction to Metallic Arsenic and Subsequent Oxidation 

 to Arsenic Trioxide. 



The powdered material is mixed with a small quantity 

 of anhydrous potassium ferrocyanide and introduced into a 

 thin walled tube of hard glass drawn down to a point and fused. 

 The tube is tapped gently to cause all the material to collect in 

 the tip of the tube. Heat the material gently at first and finally 

 raise the temperature to a red heat. The arsenical compound is 

 reduced; arsenic is set free and condenses upon the walls of the 

 tube as a brownish mirror. Antimony will yield a black or metal- 

 lic mirror; mercury a sublimate of tiny silvery spheres. Certain 

 compounds of carbon or sulphur may yield deposits upon the 

 glass closely resembling the arsenic mirror. It is therefore 

 essential to carry the test a step farther; to this end, cut off the 

 closed tip of the tube and heat the mirror over the micro-flame. 

 The arsenic will be vaporized and oxidized, collecting upon the 

 cool walls as As 2 3 in the form of glistening colorless highly re- 

 fractive (n = 1.755) isometric crystals in the form of octahedra 

 or as derivatives of the octahedron. These crystals are soluble in 

 potassium hydroxide solutions and are precipitated therefrom 

 in the form of octahedra by strong nitric acid. 



ARSENATES. 



By Means of Silver Nitrate. 



Apply reagent by Method /, page 299, to the ammoniacal 

 drop. 



This reaction has already been discussed at length under 

 Silver, Method C, page 383. 



Well-developed crystals are rarely obtained. An amorphous 



