406 ELEMENTARY CHEMICAL MICROSCOPY 



to small bulk, acidified with acetic acid, a trace of sulphuric 

 acid added and into the drop, a drop of silver nitrate is caused 

 to flow. Silver sulphate will first separate in its characteristic 

 form but will be colored yellow or red through the solid solution 

 of silver chromate in it. Later the red-brown or blackish crys- 

 tals of silver chromate appear. 



EXPERIMENTS. 



a. Look over notebook records of experiments made under Silver — Exps., 

 Method B, page 382. Similar crystals will be obtained upon testing for Cr with 

 AgN0 3 . 



b. Test for Cr in several different Cr compounds by Method B. 



c. Test by Method B in Cr salts, mixed with Al, Fe, Cu, Ni. 



d. Test for Cr in chrome iron. 



MANGANESE. 



Crystal Forms and Optical Properties of Common Salts 

 of Manganese. 



A. ISOTROPIC. 



B. ANISOTROPIC. 



Hexagonal. 



Tetragonal. 



Orthorhombic. — Potassium permanganate. 



Monoclinic. — Acetate (ous) ; chloride (ous) ; am- 

 monium-manganous sulphate; potassium- 

 manganous sulphate ; sodium-manganous 

 sulphate. 



Triclinic. — Sulphate (ous). 



DETECTION. 



A. With Manganons Salts Oxalic Acid or Primary Potassium 

 Oxalate forms Characteristic Crystals of Manganous Oxalate. 



Obtain a thin uniform film of dry potassium oxalate upon 

 the slide; Method IV, page 303. Draw across this film the 

 neutral solution of the material to be tested or a solution slightly 

 acidified with acetic acid. Six-armed stars of MnC 2 4 • 3 H 2 

 separate. These stars result from the intersection of thin twinned 

 prisms. They polarize strongly, extinguish parallel to their 

 length and exhibit brilliant polarization colors. 



