Not many of the elements occur in nature as pure substances, lying around waiting for someone to pick them up. A few do, and it is not surprising that these were known and collected for various uses in societies around the globe as far back as several thousand years B. C. Sulfur is one of those elements. It was known to burn with smelly results and have an odd appearance. Its use was probably confined to religious ceremonies. Carbon was also known since antiquity, because the charred (烧焦的) bones of animals and portions of partially burned trees consist largely of carbon in the form of charcoal. Although we don’’t know all the uses primitive peoples made of charcoal, we do know that it was the key to releasing many other elements from their chemical combination in rocks. For example, if a copper-containing rock was heated in a hot fire with charcoal present, the carbon in the charcoal would combine with the other elements in the rock, leaving free metallic copper. In ways like this people were able to discover the elements copper, iron, lead, tin, and zinc, although they didn’’t necessarily appreciate that these substances were elementary. They just knew that they were useful. Iron also arrived occasionally from the heavens in the form of iron-containing meteorites (陨石). Because of this, one might think that iron would have been thought of as a "heavenly" element, a gift from the gods. Instead, this honor has always fallen to the element gold. Gold doesn’’t fall from the skies, but it is found in its pure state in some places. It has the unusual property of never tarnishing (锈蚀) like other metals do. Gold objects don’’t rust, as iron does, nor do they turn green or black on the suce as do many other metals. The ancient Egyptians used gold for jewelry and for coins. Gold is still used for jewelry, although pure gold is too soft for this purpose. Coins and jewelry, instead, are made of alloys. These are metals made by melting two or more metals in a pot together in such a way that they dissolve in one another. Gold coins are usually 90 per cent copper. An alloy of gold and silver is called white gold. What is the purpose of Sentence 3 in Paragraph 1