ZINC ORES. 275 



at bottom, to which a sheet iron tube (k) is adapted, which tube extend* 

 down to small vessels of water, or condensers, (I, I) ; and the sole of the 

 hearth is perforated accordingly below each crucible. If one of the tubes 

 becomes clogged with metal, it is cleared by a hot iron bar. In charg- 

 ing, the hole in the bottom of the crucible is stopped by a wooden plug, 

 which afterwards becomes reduced to charcoal by the heat. The pots 

 are charged and cleared out through holes (d, d, d, d) in the cupola (a.) 

 The covers (of fire-tile, m) are placed on whenever a blue flame begins 

 to appear, as this indicates the vaporization of the zinc. 



The fire is made on the grate e, through the door/; g is the ash-pit 

 below ; m, m, m, m, in figure 2, show the position of the pots as seen in 

 a bird's-eye view. The smoke escapes from the oven by the apertures 

 d, (fig. 1,) into a conical chimney, (b,) by which a strong draught is 

 kept up. In this chimney there are as many doors (c, c, c, c) as there 

 are pots ; and in the cupola there are the same number of openings for 

 inserting or removing the pots, which are afterwards closed up by 

 brickwork ; the pots are many times refilled without removal. The 

 refuse after an operation, is shaken out through the hole in the bottom 

 of each pot, after the tube k is removed. 



The zinc as it is reduced, rises in vapor and passes down the tubes into 

 the condensers, where it collects in drops or powder with some oxyd ; 

 the metal is afterwards melted and cast into bars ; and the oxyd which 

 is skimmed off is returned to the crucibles. A charge occupies about 

 three days, and the ore affords from 25 to 40 per cent, of zinc. 



In Liege, where the ore from Altenberg is reduced, the ore is heated 

 in horizontal earthen tubes, 3 feet long and 4 to 6 inches in diameter 

 set thickly across a furnace, and around which the heat circulates. 

 From the description given, it is obvious how the process might be 

 varied, and larger combinations of pots or tubes arranged. 



The blende is roasted in a reverberatory furnace, 8 or 10 feet square. 

 the ore being placed in the furnace several inches deep, and kept con- 

 stantly stirred for 10 or 12 hours. The roasted ore is then reduced in 

 crucibles in the same manner as above explained. In England, the 

 roasted blende is mixed with as much calcined calamine and twice the 

 quantity of charcoal. 



The annual production of zinc in different countries is as follows : 



Great Britain, 1,000 tons. 



Upper Silesia and Poland, .... 36,000 " 



Belgium, 16,000 " 



Carinthia, 1,500 « 



United States, ... . 5,000 « 



Brass is made directly from the ore by heating copper with calcined 

 calamine and charcoal. At Holywell, England, 40 pounds of copper 

 and 60 of calamine yield about 60 pounds of brass. It is also made 

 from copper and roasted blende, but the product is less pure. Dr. Jack- 

 ton states that he has obtained brass of an inferior quality by heating 

 ogether in a crucible copper pyrites and blende after roasting them. 

 Brass is commonly made in this country by melting together the raetala 

 inc and copper. 



How is blende reduced 1 How is brass made 1 



