SOME CHEMISTRY OF EVERYDAY LIFE 203 



have been prepared, or the cotton for the cloth has been 

 grown, or the wool to be used has been clipped from the 

 backs of sheep. 



Iron when heated softens long before it melts, and while 

 hot can be drawn into wire or be rolled into sheets and other 

 desired forms. In the big rolling mills steel rails for rail- 

 roads, and the great pieces of structural iron for bridges and 

 for the framework of big office buildings in cities, are rapidly 

 handled and given the forms desired by means of powerful 

 machinery in use there. The shaping of iron on the anvil 

 of the blacksmith when hot, and the welding of iron whereby 

 two pieces become one, is made possible by the fact that 

 iron softens without melting as it is heated. 



Exposed to air and moisture iron surfaces become corroded 

 with "rust" (iron oxide). This can be prevented by keep- 

 ing the iron painted, or by means of a thin coating of some 

 metal not readily affected by the atmosphere. The " tin- 

 plate" of dealers in metals used for roofing, kitchen utensils, 

 and dairy articles, is sheet iron which has been dipped into 

 molten tin, a very thin coat of which adheres to the iron. 

 The "galvanized" iron ware is sheet iron, wire, etc., which 

 has been similarly dipped into molten zinc which forms a 

 protective coating on the iron. 



By heating iron and steel to certain temperatures deter- 

 mined by experiment and recognized by the skilled worker in 

 metals by the color, and then cooled at a rate and in a 

 manner likewise learned by experience, certain desired 

 degrees of hardness, elasticity, flexibility, and tenacity may 

 be given these metals in a process known as tempering. 

 Edged tools of all kinds, as swords, razors, knife blades, 

 chisels and axes must be made of tempered steel. The metal 

 must be sufficiently hard to hold an edge when ground, but 

 not so brittle as to break easily when hard objects are 

 cut. It must be flexible enough to bend without breaking, 



