l6o OUR PHYSICAL WORLD 



oxygen of the ore unites with the carbon present as charcoal or 

 coke and forms gaseous oxides of carbon. Some impurities in 

 the ore such as phosphorus and sulphur also unite with the 

 oxygen to form their oxides, also gases, while others like silicon 

 unite with the limestone or "flux" and form a glassy slag. 



It was found as the furnace stack was made larger that the 

 melted iron because of its weight sank to the bottom of the stack 

 while the melted slag, being lighter, floated on top of it. The slag 

 could be drawn off, and then the iron through a lower hole, and so 

 the furnace could be run continuously instead of letting the fires go 

 out to get the iron. The stack came to be larger and larger, 

 was built of brick and lined with firebrick, and the bellows was 

 operated by power. Still later a rotary fan was used to blow 

 the draft into a furnace. The ores, flux, and charcoal were taken 

 to the top of the stack by elevator, piled on a movable lid on 

 the top of the stack, and fed into the stack when this lid was 

 opened by machine power. When the iron was drawn off it 

 was run into a trough in the molding-sand floor adjacent to the 

 stack, and from this was led into small tributary troughs where it 

 hardened into "pigs," so called because they lay side by side 

 like a row of nursing pigs. Iron thus produced was called pig 

 iron (Fig. 64). 



There was a vent from such a stack carrying off into the air 

 the inflammable gases. Now these are brought by great pipes 

 down under the boilers to make steam for power to handle the 

 ore, flux, and charcoal or coke, and under great steel stoves that 

 heat the air to be driven into the furnace so the fires in the stack 

 may not be cooled by the entrance of cold air. The pipes that 

 carry this hot-air blast into the furnace have their points cooled 

 by a jacket of constantly changing water so that they will not melt 

 in the intense heat. The ore, flux, and fuel are handled by 

 machinery, so that human labor is reduced to a muiimum. 



All the products which distil off as the wood isheated in the char- 

 coal kilns (Fig. 65) or the coal is made into coke in the ovens, and 

 which at one time were turned into the air as wastes, are now 



