History

The origin of modern atomic theory - Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808)

Atomism
The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny quantities has been around for millennia, but these ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. The nature of atoms in philosophy varied considerably over time and between cultures and schools, and often had spiritual elements. Nevertheless, the basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.
The earliest references to the concept of atoms date back to ancient India in the 6th century BCE, appearing first in Jainism. The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects. In the West, the references to atoms emerged a century later from Leucippus, whose student, Democritus, systematized his views. In approximately 450 BCE, Democritus coined the term átomos (Greek: ἄτομος), which means "uncuttable" or "the smallest indivisible particle of matter". Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.
Corpuscularianism is the postulate, expounded in the 13th-century by the alchemist Pseudo-Geber (Geber), that all physical bodies possess an inner and outer layer of minute particles or corpuscles. Corpuscularianism is similar to the theory atomism, except that where atoms were supposed to be indivisible, corpuscles could in principle be divided. In this manner, for example, it was theorized that mercury could penetrate into metals and modify their inner structure. Corpuscularianism stayed a dominant theory over the next several hundred years and was blended with alchemy by Robert Boyle and Isaac Newton in the 17th century. It was used by Newton, for instance, in his development of the corpuscular theory of light.