362 ELEMENTARY CHEMICAL MICROSCOPY 



bristling masses and tiny crosses and stars, closely resembling 

 the forms obtained with cadmium. They differ, however, from 

 the cadmium salt in that they do not polarize. 



Nickel gives a light green amorphous precipitate; cobalt a 

 similar pink one; while iron, if heated, yields a yellow deposit. 



Mercurous salts (nitrate) give a gelatinous amorphous mass 

 of a yellowish tint. 



Mercuric salts and those of silver, lead, tin, antimony, bismuth, 

 aluminum, magnesium and the alkaline earths appear to give 

 no precipitates and to yield no crystals even in concentrated 

 solution or upon evaporation. 



Precautions. 



The solution should be neutral or but faintly acid and should 

 be moderately concentrated with respect to zinc. 



If no result is obtained upon the first test, make a second, 

 employing a considerably greater amount of the unknown 

 substance. 



Heating the preparation hastens the reaction. 



If a precipitate is obtained, zinc, cadmium, copper, nickel, 

 cobalt, iron or manganese are present and, conversely, if no pre- 

 cipitate appears, these elements must be absent. 



Sodium nitroprusside is thus a convenient group reagent. 



EXPERIMENTS. 



a. Try the reagent upon several different concentrations of Zn. 



b. Try with Cd, then with mixtures of Zn and Cd. 



c. Try salts of Cu, Ni, Co, Mn, first as pure salts, then as mixtures with Zn. 



CADMIUM. 



Crystal Forms and Optical Properties of Common Salts 

 of Cadmium. 



A. ISOTROPIC. 



B. ANISOTROPIC. 



Hexagonal. — Iodide, ammonium-cadmium bro- 

 mide; ammonium-cadmium chloride; potas- 

 sium-cadmium chloride. 



