364 ELEMENTARY CHEMICAL MICROSCOPY 



Precautions. 



Cadmium salts of the organic acids, as, for example, cadmium 

 acetate, fail to yield a satisfactory rest. It is therefore best to 

 evaporate the unknown with nitric acid and drive off the excess 

 of acid before adding the thiocyanate reagent. It follows that 

 the addition of sodium or ammonium acetate to very acid solu- 

 tions to lessen the effect of the mineral acid is in this case unwise. 

 It is better to evaporate to dryness. 



B. By Means of Oxalic Acid. 

 Read Method C, Zinc, page 359. 



The typical crystals of cadmium oxalate CdC 2 4 • 3 H 2 con- 

 sist of long, clear, colorless, monoclinic prisms, singly, in X's, 

 or in clusters. The obliquely truncated ends constitute a dis- 

 tinctive feature. 



Manganous oxalate MnC 2 4 • 3 H 2 separates in groups of 

 radiating prisms, which the careless observer sometimes con- 

 fuses with the cadmium salt or vice versa. The ends of the 

 prisms of the two salts are quite different however in appearance. 



C. By Means of Sodium Nitroprusside. 

 See Zinc, Method D, page 361. 



MERCURY. 



Crystal Forms and Optical Properties of Common Salts 

 of Mercury. 



A. ISOTROPIC. 



B. ANISOTROPIC. 



Hexagonal. 



Tetragonal. - - Mercurous bromide, chloride and 



iodide; mercuric cyanide; red mercuric 



iodide. 

 Orthorhombic. — Mercuric bromide ; mercuric chlo- 

 ride; yellow mercuric iodide. 



