180 METALS AND THEIR COMBINATIONS. 



7. Potassium or sodium hydroxide causes a similar green precipi- 

 tate of chromic hydroxide, which is soluble in an excess of the 

 reagent, but is re-precipitated on boiling for a few minutes. 



c. Of chromium in any form. 



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

 sium) carbonate and nitrate, give, when heated upon platinum foi4, a 

 yellow mass of the alkali chromate. 



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

 bead. 



Cobalt and Nickel, Co == 58.7, Ni = 58.6. 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. 



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.) 



27. ZINC. 

 Znii = 65.1. 



Occurrence in nature. Zinc is found chiefly either as sulphide 

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

 in combination with silicic acid as silicate and with oxygen as the red 

 oxide. 



Metallic Zinc is obtained by heating in retorts the oxide or 

 carbonate mixed with charcoal, when decomposition takes place. 



QUESTIONS. 251. How is manganese found in nature? 252. Mention the 

 different oxides of manganese. What is the binoxide used for ? 253. What 

 is the color of manganese salts, of manganates, and of permanganates ? 254. 

 How is potassium permanganate made ; what are its properties, and what is it 

 used for ? 255. Give tests for manganese. 256. State composition and prop- 

 erties of potassium dichromate. 257. How is chromium trioxide made ; what 

 are its properties ; what is it used for ; and under what other name is it known ? 



258. By what process may chromium sesquioxide be converted into chromates? 



259. What is the composition of the oxide and hydroxide of chromium, and 

 how are they made? 260. Mention tests for chromates and chromium salts. 



