Atomic hypothesis


 * If, in some cataclysm, all scientific knowledge were to be destroyed, and only one sentence passed on to the next generation, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or atomic fact, or whatever you wish to call it) that all things are made of atoms--little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see there is an enormous amount of information about the world, if just a little imagination and thinking are applied. (Richard Feynman, 1963)

Historical development
The concept that that everything is made of atoms is quite old. Democritus in the 5th century BC proposed it, it was later picked up and discussed by Epicurus in the late 4th to early 3rd century BC, but it was just an idea that was never developed or really accepted. Aristotle rejected the idea since he thought it was necessarily associated with a void which he also rejected. Pierre Gassendi in the late 16th and early 17th century AD wrote criticising Aristotle and is widely acknowledge as the one who revived the idea. He described his approach to this idea in a book he published in 1649 AD. He was of the opinion that the properties of atoms depneded on their shape. He also thought that they might join together and form molecules. In addition he proposed that atoms moved in a void with nothing between them.