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 A rsenic 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 AS2O3 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 I, 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 
