Page:General History of Europe 1921.djvu/792

592 General History of Europe 1076. The Atomic Theory. While the zoologist, the botanist, and the geologist were elaborating the theory of evolution, the chemists, physicists, and astronomers were busy with the problems suggested by matter and energy heat, light, electricity, the nature and history of the sun and stars. Early in the nineteenth century an Englishman, Dalton, suggested that all matter acted as if it consisted of atoms of the various elements, which combined with one another to form the molecules, or little particles of the innumerable compound substances. This theory, when carefully worked out, became the foundation of modern chemistry. The chemist was long satisfied with the idea that the atoms were the smallest particles of matter which existed. He gradually added to the list of various kinds of atoms until he had about eighty elements, as he named them, out of which all things appear to be composed. But the idea that the atom is the smallest possible particle of matter has had to be given up. Early in the twentieth century it was discovered that radium, an exceedingly rare and precious clement, had the peculiarity, along with some other very heavy atoms, of breaking up into far smaller particles, called electrons. So it is now supposed that all atoms are made up of electrons rapidly vibrating about a nucleus. The electrons act like charges of negative electricity. There is therefore no such thing as " dead " matter, for the movements of the electrons, atoms, and molecules in what seems to us a cold, inert stone are so incredibly rapid and complicated as to defy description. 1077. Light and Electricity. During the nineteenth century the nature of heat and light was at last explained. Light and radiant heat are transmitted by minute waves produced, it is supposed by many scientists, in the ether, a something which it is assumed must everywhere exist, for without some medium the light would not reach us from the sun and stars. Electricity, of which very little was known in the eighteenth century, has now been promoted to the most important place in the physical universe. Light is believed to be nothing more than electric forces traveling through the ether from a source of