Now we come to steel, the most wonderful product or form in which we take advantage of the value of iron. Steel was formerly made from wrought-iron, so that you first had to get cast-iron, from which you made wrought-iron, and eventually got steel by changing the wrought-iron. Now we make steel direct from pig-iron. This is known as the Bessemer process. The most noticeable feature in the chemical composition of the different grades of iron and steel is found in the percentages of carbon they contain. Pig-iron contains the most carbon; steel the next lowest, and wrought-iron the least. Iron has been known to men from early historical times. The smelting of iron ores is not any indication of advanced civilization either. Savage tribes in many parts of the world practiced the art of smelting, even before they could have learned it from people who had become civilized. Why Is Gold Called Precious? Gold is called one of the precious metals because of its beautiful color, its luster, and the fact that it does not rust or tarnish when exposed to the air. It is the most ductile (can be stretched out into the thinnest wire), and is also the most malleable (can be hammered out into the thinnest sheet). It can be hammered into leaves so thin that light will pass through them. Pure gold is so soft that it cannot be used in that form in making gold coins or in making jewelry. Other substances, generally copper, are added to it to make the gold coins and jewelry hard. Sometimes silver is also added to the gold with copper. The gold coins of the United States are made of nine parts of gold to one of copper. The coins of France are the same, while the coins of England are made of eleven parts of gold to one of copper. The gold used for jewels and watch-cases varies from eight or nine to eighteen carats fine. Another reason why gold is called a precious metal is that it is very difficult to dissolve it. None of the acids alone will dissolve gold, and only two of them when mixed together will do so. These are nitric acid and hydrochloric acid. When these two acids are mixed and gold put into the mixture the gold will disappear. What Do We Mean By 18-Carat Fine? We often hear people in speaking of their watches say, “It is an 18-carat case.” Others speak of 14-carat watches or 22-carat or solid-gold rings.