We now know of more than one hundred elements. In the year 1800, about half that number had been discovered. And it was at about this time that an Englishman, John Dalton, came along with the first really scientific _atomic theory_. Dalton said that all elements are made of atoms, and the atoms of any particular element are always the same. An atom of carbon is always like every other atom of carbon. And, Dalton went on, atoms of different elements have different weights. An atom of carbon weighs more than an atom of hydrogen. An atom of oxygen weighs more than an atom of carbon. Dalton noticed that atoms combined in different ways according to their weights. Water is made of hydrogen and oxygen. But it takes _two_ hydrogen atoms with _one_ oxygen atom to make water. This smallest unit of water is called a _molecule_. A molecule is the smallest possible amount of any chemical compound. Another contribution that Dalton made was the use of shorthand symbols to stand for chemical elements. Today, we use a system which grew out of Dalton's. [Illustration] Instead of calling the water molecule "two atoms of hydrogen and one atom of oxygen," we call hydrogen "H" and oxygen "O" and write, simply, H_{2}O. The symbol for carbon is "C" and carbon dioxide may be written CO_{2}. This means that one carbon atom and two oxygen atoms combine to form one molecule of carbon dioxide. John Dalton was a great scientist and almost everything in his atomic theory turned out to be correct. Only one of his ideas we now know to be wrong. And that idea went all the way back to Democritus. They both thought that the atom was the smallest possible particle, and that it could never be divided. It certainly seemed to be so. But astonishing things about atoms began to turn up around the end of the 1800's. No one knew it yet, but men were going to change atoms--and atoms were going to change the world! 3 THE MYSTERY OF THE RAYS In 1895, a German scientist, Wilhelm K. Roentgen (RENT-gen), was experimenting with a special kind of electrical tube. He had covered one end of the tube with black paper. Nearby was a screen that glowed when light shone on it. Roentgen happened to glance at the screen--and could hardly believe his eyes. It was glowing--but there was no light coining from the covered tube! This seemed impossible--but there it was. Later, when someone asked Roentgen what he thought, he made a true scientist's reply. He said, "I did not think. I investigated!" Roentgen didn't know what the rays were, so he decided to call them _X rays_. He found that they could pass not only through black paper, but through many other substances as well. They went easily through cloth or wood, but were slopped by metal. Roentgen found that the mysterious rays could shine right through the soft parts of the body, but they were stopped by bones. [Illustration] [Illustration]