INDUSTRIAL MINERALS DIAGRAM 20. 231 



suddenly cooled. This process is called tempering. The uses of 

 cast-iron, iron, and steel, are innumerable. 



Loadstone is a kind of iron ore which possesses the property of 

 attracting iron to it. When bars of soft iron are rubbed with it, 

 they acquire the same property, and are then called magnets. 

 Loadstone is not, however, the only body which thus attracts 

 others. It is enough to rub a stick of sealing wax on cloth, and 

 put it near the down of a feather or a very small piece of paper, 

 to see these very light bodies attracted as is iron by the magnet. 

 This, however, is produced by electricity, and not by mag- 

 netism. 



When a piece of soft iron has thus been rendered magnetic, we 

 find that one end attracts and the other repels iron. If the 

 magnet is then suspended so that it can move freely, we shall 

 find that it will move round until the attracting or positive pole 

 points to the north, and the repelling or negative pole to the south. 

 The mariner's compass, by which ships are guided across the sea, 

 is formed of a needle which has thus been rendered magnetic, and 

 always points north and south. 



Copper Ore. Copper, like iron, is rarely met with in a pure 

 state, but is nearly always mixed with sulphur. The treatment 

 of copper ore is rather tedious. When it has been washed and 

 crushed, it is roasted several times to burn the sulphur. It is 

 then melted, and the copper separates. Copper is of a red 

 colour, and is used to make boilers, saucepans, and a variety of 

 utensils, but they ought always to be kept very clean, as other- 

 wise verdigris forms in them, which is a violent poison. To avoid 

 this inconvenience, the inside of copper vessels which are not 

 required to be exposed to a great heat, are tinned, but they are 

 useless if too much heated, as the tinning melts. 



When copper and tin are mixed together in proper proportions, 

 we obtain bronze, which is more durable than copper, and much 

 less easily tarnished in the air. Bronze is used to make statues, 

 cannons, and a great many objects which are very little injured by 

 time. 



