The most satisfactory way to classify elements is to consider them as metals or non-metals. Non-metallic elements are those that combine readily with metals to form compounds; for example, chlorine, sulphur, silicon, phosphorus, etc. The non-metallic elements have not so much trade importance 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.