The ancient Greeks had the idea that everything on earth was made up of four simple substances—fire, air, earth, and water. A part of their idea was right. All the millions of different materials in the world are made of certain simple sub­stances. We call them elements. They can be thought of as the building blocks of the universe. But the Greeks were wrong about what the simple substances are and how many there are.

   There are a few more than 100 elements. For many years scientists felt sure that there were just 92. There are just 92 natu­ral elements. But scientists have produced several others in their laboratories.

   More than three-fourths of all the ele­ments are solids. Most of the others are gases. Carbon and iron are examples of solid elements. Oxygen and chlorine are gases. There are only two liquid elements. They are mercury and bromine.

   The symbols for most elements are easy to understand. It is easy to see why C was chosen to stand for carbon and O for oxygen. But it is not easy to see why Fe stands for iron and Au for gold. These symbols and others like them were made from the Latin names of the elements they stand for. Ferrum is the Latin word for iron. Aurum is the Latin word for gold.

   The story of chemistry is partly a story of the discovery of one element after another. Many scientists made themselves famous by discovering an element. The English scientist Priestley, for example, is famous for his discovery of oxygen. Many elements were hard to discover for they are never found free. They are always, that is, joined with other elements.

   One element, helium, was found in the Sun before it was found on Earth. Its name comes from the Greek word for Sun. Scien­tists discovered it by studying sunlight. It is no wonder that helium was found first in the Sun. It is rather rare on Earth, while almost half the Sun is made up of it. Almost all the rest of the Sun is hydrogen. Other elements make up only about one one-hundredth of the Sun.

   The same story is true of the whole universe. The many billions of stars are mostly hydrogen and helium. There is 99 times as much hydrogen and helium in the universe as of all the other elements put together.