MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 351 



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 A^Oa 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 251, to the ammoniacal drop. 



This reaction has already been discussed at length under Silver, 

 Method C, page 337. 



By Means of Zinc Chloride in Ammoniacal Solution. 



To the test drop add ammonium hydroxide, then apply the zinc 

 chloride by Method 7, page 251. 



Ammonium zinc arsenate NKtZnAsC^ 6 H 2 O separates in 

 the same forms as those described for ammonium magnesium 

 phosphate (q. v.) with which it is isomorphous, as also with the 

 compound NHiZnPC^ 6 H 2 O ; but the crystallites of the am- 

 monium zinc arsenate are more feathery than those of the 

 ammonium magnesium phosphate. Phosphates must be absent. 



ARSENITES. 



By Means of Silver Nitrate. 



Apply the reagent by Method /, page 251, to the ammoniacal 

 drop. 



Lemon yellow silver arsenite is immediately precipitated first 

 as an amorphous mass, later crystallizing in a variety of forms. 

 The first crystals appear as exceedingly tiny acicular crystals in 

 masses, stars and crosses, later as fusiform grains, and still later 

 as thin rods with notched ends, or long irregular acicular prisms. 

 Eventually some oxidation takes place and there will appear 

 crystals of silver arsenate. Silver arsenite is soluble in acids 

 and in ammonium hydroxide, hence the amorphous precipitate 

 partially redissolves. 



