128 APPLIED SCIENCE 



as the metals and consequently will not be considered in 

 detail in this book. 



Metals are good conductors of heat, that is, warmth or 

 heat travels rapidly through them. About one-half of all 

 the known metals are very scarce, and some of them have 

 been seen by only a few persons. A few of the metals, like 

 gold, platinum, silver, copper, and bismuth are found in 

 a free state, that is, pure and unmixed with other materials. 

 The majority of the metals are found in ores combined with 

 oxygen or sulphur. 



The art of extracting these metals from their ores and 

 refining them is called metallurgy. This extraction may be 

 accomplished in two ways: by the dry method, and by the 

 wet method. In the dry process, the metal is separated 

 from its ore by heat, and the use of high temperature in 

 large furnaces of different kinds is involved. This is the 

 process used in extracting pig iron (see page 370) from iron 

 ore. The wet method involves crushing or pulverizing 

 the ore, as in the case of copper ore, and treating it with 

 chemical liquids and acids, through which an electric current 

 is passed. This latter method is known as the electrolytic 

 process and involves what is termed electrolysis. 



133. Atomic Weight. Atoms are assumed to have a 

 definite weight. Hydrogen is the lightest element and has 

 therefore been selected as the unit of weight; all other ele- 

 ments are measured in terms of hydrogen. For example: 

 If equal volumes of hydrogen and oxygen are weighed, 

 oxygen is found to weigh sixteen times as much as hydrogen. 

 Hence the atomic weight of oxygen is 16. 



The atomic weights of the commonest elements are given 

 in the following table: 



