By the early years of the 1900's, some people were beginning to worry about fuel. There wasn't any shortage yet, and there wouldn't be for a long time. There were still new coal mines and new oil wells being discovered. But some day, the last one _would_ be discovered, the fuels would be all used--and then what? [Illustration] As new industries developed all over the world, the need for power grew--and so did the worry. And then suddenly, in 1942, one of the greatest discoveries of all time was made. The atomic age was born and the perfect fuel was found. Atomic energy can do things never before dreamed of. Atoms, without any fire, without any noise, can run a huge electricity plant. The atoms in one pound of fuel can keep the lights burning in your home for 1000 years! Atoms can send a large ship back and forth across the ocean many times--on one small load of fuel! Atoms can keep hamburger fresh for weeks without freezing! Atoms can sometimes cure sick people! Atoms can find leaks in pipes, and can test the rubber in a set of car tires! Atoms, in many ways, can make life easier, healthier, and better for us. What are these wonderful atoms, anyway? Let's find out. [Illustration] 2 THE SMALLEST THING THERE IS The story of atoms begins about two thousand years ago, in ancient Greece. There, a wise man named Democritus started to think about what things were made of. Democritus reasoned that you could take a little piece of anything at all--wood, metal, even candy--and cut it in half. Then you could cut the halves in half. Then you could cut _those_ pieces in half. And so on. But finally, Democritus said, you would get to particles so tiny that they could not be divided any more. He called these smallest particles _atoms_, from the Greek word _atomos_ which means _indivisible_. It was hard for people to believe that everything in the world was made of atoms. They soon forgot about Democritus and his strange ideas. Atoms were forgotten for almost two thousand years. But during the eighteenth century, European scientists again became interested in the structure of things and began to experiment. There were some curious discoveries. [Illustration] Chemists found that a substance like water could be separated into two other substances, _hydrogen_ and _oxygen_. But hydrogen and oxygen couldn't be broken down into any other chemicals no matter what the chemists did. They could easily change table salt into _sodium_ and _chlorine_. But no matter how hard they tried, they weren't able to break down sodium and chlorine into anything else. So scientists decided that most of the things around us--water, salt, wood, animals--were made of combinations of substances. They called the combinations _chemical compounds_. They called the substances that couldn't be broken down _elements_.