COMMON ELEMENTS OF THE EARTH 153 



cast iron contains a larger per cent of carbon and some other 

 impurities. In these three forms, iron has certain properties 

 in very different degrees; wrought iron is most malleable, 

 steel most tenacious and elastic, cast iron most brittle. 

 Chains are made of wrought iron; car rails and knife blades 

 are made of steel; heavy pieces of machinery are made of 

 cast iron. Iron rusts if exposed to outdoor air; to prevent 

 rusting it is often covered with paint, zinc, or graphite in the 

 form of " blacking. 



168. Copper and Gold. Copper and gold are the only 

 well-known metals that occur in abundance free from 

 other elements. They are said to occur free or native. 

 Copper and gold have characteristic colors, copper being dull 

 red, and gold, yellow; most of the other metals have a gray- 

 ish color. Both copper and gold are rather soft, have a 

 high melting point, and are malleable and ductile in a very 

 high degree. They have both been used since early times. 

 This was because they were found free and required no 

 difficult process of separation from the ore, and because 

 they could be hammered into shape with simple tools. 



Gold is less affected by the chemical action of air and water 

 than almost any other metal, and it is not acted upon readily 

 by any common substance. Hence it is used the world over 

 for coins and jewelry. As gold is too soft to be used pure 

 for such purposes, a little copper or silver is melted with it 

 to give it greater hardness. Such a mixture of metals is 

 called an alloy. Gold that is described as "24 carat" is 

 100 per cent pure gold. The best grades of jewelry are 18 

 carat gold, while the 14 carat is used for articles which are 

 likely to have hard wear. United States gold coins are 90 

 per cent gold and 10 per cent copper; they contain an 

 amount of gold equal in value to the value of the coin. 



Because of its great malleability, gold can be hammered 

 so thin that 250,000 sheets of gold leaf are required to make 

 a pile one inch thick. It is much thicker than that, how- 



