312 METALS AND THEIR COMBINATIONS. 



c. Of chromium in any form. 



9. Compounds of chromium, when mixed with sodium (or potas- 

 sium) carbonate and nitrate, give, when heated upon platinum foil or 

 in a crucible, a yellow mass of the alkali chromate. 



10. Compounds of chromium impart a green color to the borax 

 bead. Use only a very small quantity of the chromium compound. 



Chromium salts have a green or violet to purple color. Solutions 

 of the violet salts turn green when heated. They are acid to litmus, 

 due to hydrolysis in solution. Chromates are all red or yellow, and 

 mostly insoluble in water. The color of a chromate is noticeable in 

 very dilute solution (made with the aid of an acid in the case of in- 

 soluble salts). 



Cobalt and Nickel, Co =58.56, Ni = 58.3. These two metals show much 

 resemblance to each other in their chemical and physical properties, and occur 

 in nature often associated with each other as sulphides or arsenides. 



Both metals are nearly silver-white ; the salts of cobalt show generally a red, 

 those of nickel a green color. The solutions of both metals give a black pre- 

 cipitate of the respective sulphides on the addition of ammonium sulphide. 

 Ammonium hydroxide produces in solutions of cobalt a blue, in solutions of 

 nickel a green precipitate of the hydroxides, both of which are soluble in an 

 excess of the reagent ; potassium or sodium hydroxide produces similar pre- 

 cipitates, which are insoluble in an excess. Sodium carbonate produces in 

 solutions of cobalt a violet, and in solutions of nickel a green precipitate of the 

 respective carbonates. (Plate I., 7 and 8.) 



Cobalt is used chiefly when in a state of combination (for coloring glass blue) ; 

 nickel when in the metallic state. (German silver is an alloy of nickel, copper, 

 and zinc.) 



29. ZINC. 

 Zn == 64.9. 



Occurrence in nature. Zinc is found chiefly either as sulphide 

 (zinc-blende), ZnS, or as carbonate (calamine), ZnCO 3 ; it occurs also 

 as silicate, H 2 Zn 2 SiO 5 , and as oxide in combination with the oxides 

 of iron or manganese. 



QUESTIONS. How is manganese found in nature? Mention the different 

 oxides of manganese. What is the dioxide used for ? What is the color of 

 manganese salts, of manganates, and of permanganates? How is potassium 

 permanganate made; what are its properties, and what is it used for? Give 

 tests for manganese. State composition and properties of potassium dichro- 

 mate. How is chromium trioxide made ; what are its properties ; what is it 

 used for ; and under what other name is it known ? By what process may 

 chromium sesquioxide be converted into chromates ? What is the composition 

 of the oxide and hydroxide of chromium, and how are they made? Mention 

 tests for chromates and chromium salts. 



