350 ELEMENTARY CHEMICAL MICROSCOPY 



Elements of the magnesium group interfere. Lithium like- 

 wise interferes. But the chlorides of iron and aluminum and 

 the salts of boric acid have no appreciable effect on the reaction. 



When in doubt as to the nature of a precipitate formed by the 

 treatment with HNaC0 3 , decant the supernatant solution, which 

 is easily done since the crystals of calcium carbonate adhere to 

 the glass slide, wash the residue, and then add dilute sulphuric 

 acid. If the precipitate is due to calcium, characteristic crys- 

 tals of CaS0 4 • 2 H 2 appear. 



Primary sodium carbonate is of more value as a group reagent 

 than as an identification test. Moreover, chance formations of 

 crystals of alkali carbonates may be met with in the progress 

 of the systematic analysis of unknown material, particularly 

 when testing for zinc (q.v.). 



MAGNESIUM. 



Crystal Forms and Optical Properties of Common Salts 

 of Magnesium. 



A. ISOTROPIC. 



B. ANISOTROPIC. 



Hexagonal. — Pyroantimonate. 



Tetragonal. — - Fluoride. 



Orthorhombic. — Ammonium-magnesium phosphate ; 

 sulphate; primary tartrate. 



Monoclinic. — -Acetate; chloride; nitrate; primary 

 phosphate ; ammonium-magnesium sul- 

 phate; potassium-magnesium sulphate; 

 normal tartrate. 



Triclinic. 



DETECTION. 



A. By Means of Uranyl Acetate and Sodium Acetate. 



This test has already been described at length under 

 Sodium, Method A, page 321. 



B. By Means of Secondary Sodium Phosphate {HNa> 2 POi) 

 in Ammoniacal Solution. 

 For the reaction see Ammonium, page 332. 



