ALUMINIUM. MAGNESIUM AND ZINC. 237 



331. Zinc. The principal ores of this metal are the sul- 

 phide, the silicate called calamine, the oxide, and the car- 

 bonate. In obtaining the metal, if the sulphide 



and carbonate are used, they are first roasted, the 

 heat driving off the carbonic acid from the car- 

 bonate, and the sulphur from the sulphide, in the 

 form of sulphurous anhydride. This leaves the 

 ore in the state of oxide. The ore is now mixed 

 with charcoal, and introduced into an iron cruci- 

 ble, a vertical section of which is given in Fig. Fi s- 94 

 94. The crucible is closed at the top, and has an iron tube 

 passing through a hole in the bottom, and also down 

 through the floor of the furnace in which the crucible is 

 placed. The upper opening of this tube is above the sur- 

 face of the mixed ore and charcoal, and the lower opening 

 is very near to the surface of water in a reservoir. When 

 the heat is applied the carbon, uniting with the oxygen of 

 the oxide, forms carbonic oxide and anhydride, which pass 

 out through the tube and escape. Now as the zinc is vola- 

 tile, it passes out also with them in the form of vapor, but, 

 condensing as it gets in the tube below the fire of the fur- 

 nace, it drops as a liquid into the reservoir of water, where 

 it becomes solid. Zinc is a bluish-white metal. It has but 

 a single oxide, ZnO. It takes fire when heated to a bright 

 red heat, and burns with a brilliant white flame, with a tinge 

 of green. As it burns the oxide formed flies off in flakes, 

 which the alchemists fancifully called lana philosophica, 

 philosopher's wool, and nihil album, white nothing. 



332. Carbonate of Zinc, ZnCO 3 . The thin whitish film 

 which forms over the surface of zinc by exposure to air is 

 a carbonate of zinc, the water and the carbonic anhydride 

 of the air both entering into its composition. The carbon- 

 ate of zinc, under the name of smithsonite, is an important 

 ore of this metal. 



