Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory.
- Law of Conservation of Matter. Historically, already the ancient Greeks proposed the idea that the total amount of matter in the universe is constant. The principle of conservation of mass was first outlined by Mikhail Lomonosov in 1748. However, the law of conservation of matter (or the principle of mass/matter conservation) as a fundamental principle of physics was discovered by Antoine Lavoisier in the late 18th century. It was of great importance in progressing from alchemy to modern chemistry.
- Law of Definite Proportions. This law (proven by the French chemist Joseph Louis Proust) states that a given chemical compound always contains its component elements in a fixed ratio, regardless of the quantity or source of the original substance. John Dalton studied and expanded upon this previous work and developed the law of multiple proportions.
In the early 19th century, atomic theory entered the scientific mainstream when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of atoms. The modern proof for the atomic nature of matter was first proposed by the English chemist John Dalton. His theory shows that the early Greeks held the correct concept that matter consists of a combination of basic elements.
In 1803 John Dalton published the first comprehensive atomic theory in A New System of Chemical Philosophy. He proposed that the elements comprise indestructible atoms, each chemical element possesses a particular kind of atom, and all the atoms of a particular element are identical. These atoms can combine to form more complex structures (chemical compounds or molecules).