As the Middle Ages ended, alchemy gradually faded, and the science of chemistry arose. Alchemists discovered and rediscovered many facts but did not make them broadly available. Certain systematic features were recognized, but similarities between common and rare elements resulted in efforts to transmute them (lead into gold, in particular) for financial gain. Over the centuries, discoveries were made regarding the properties of substances and their chemical reactions. More than 2000 years passed before observations could be made with equipment capable of revealing the true nature of atoms. The Greeks had identified the most common examples of the four states of matter (solid, gas, plasma, and liquid), rather than the basic elements. The famous proposal that the basic elements were earth, air, fire, and water was brilliant, but incorrect.

The Greeks and others speculated about the properties of atoms, proposing that only a few types existed and that all matter was formed as various combinations of these types. The Greeks also felt that atoms were in constant motion, another correct notion. We now know that atoms themselves can be subdivided, but their identity is destroyed in the process, so the Greeks were correct in a respect. Another is what Democritus in particular believed-that there is a smallest unit that cannot be further subdivided. One is that infinitesimally small subdivision is possible.

There are only a few possible answers to this question.

(There is some evidence that philosophers in both India and China made similar speculations, at about the same time.) They considered the question of whether a substance can be divided without limit into ever smaller pieces. The earliest significant ideas to survive are due to the ancient Greeks in the fifth century BCE, especially those of the philosophers Leucippus and Democritus. People have long speculated about the structure of matter and the existence of atoms. How do we know that atoms are really there if we cannot see them with our eyes? A brief account of the progression from the proposal of atoms by the Greeks to the first direct evidence of their existence follows.

Einstein atomic theory