By the end of the eighteenth century, heat along with its cohorts light, magnetism and electricity were regarded as an imponderable fluid capable of flowing between the spaces assumed to be present in matter.
In 1789, Lavoisier showed that the total mass during the course of a chemical reaction is unchanged. Rather the atoms simply “reorganize” themselves, kind of like the reshuffling of deck or cards.
The Greek philosopher Anaxagoras considered matter to be infinitely divisible.
The first “atomic theories” focused on a “primary element” responsible for creating all other matter. Heraclitus said it was fire, Thales of Miletus (c.624 BC–c.546 BC) said it was water, Anaximenes (c.585 BC–c.528 BC) thought it was air, and Empedocles finally unified these declaring there to be the four elements of air, earth, fire and water. Later Aristotle adopted Empedocles’ four elements and so it remained up until about the 17th century.