398 ELEMENTARY CHEMICAL MICROSCOPY 



precipitate first appears, changing in part into crystalline grains 

 and yellowish or reddish brown crystallites. 



By Means of Magnesium Chloride in Ammoniacal Solution. 



To the test drop add ammonium hydroxide, then apply the 

 magnesium chloride by Method /, page 299. 



Ammonium magnesium arsenate, NH 4 MgAsO 4 -6 HoO, sepa- 

 rates in the same forms as those described for ammonium mag- 

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

 with the compounds NH 4 ZnPO 4 6 H 2 O and NH 4 ZnAsO 4 

 6 H 2 O. A little NH 4 C1 should be present in both drops. 



ARSENITES. 



By Means of Silver Nitrate. 



Apply the reagent by Method /, p. 299, 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. 



EXPERIMENTS. 



a. Test by Method A the following: solutions of As2Os; of NaAsO 2 ; of 

 H 2 KAsO 4 ; one drop of commercial H 2 SO 4 ; one drop of commercial HC1; trying 

 first the AgNO 3 crystal and then the HgBr 2 fiber. 



b. Test the above compounds by Method B. 



c. Test the same compounds with AgNOs; and finally with 



ANTIMONY. 



Crystal Forms and Optical Properties of Common Salts 

 of Antimony. 



A. ISOTROPIC. 



B. ANISOTROPIC. 



Hexagonal. Red tri-iodide; strontium-antimonyl 

 tartrate; lead-antimonyl tartrate. 



