The chemist Lavoisier actually tried to weigh heat; he found it was weightless.
With a firm belief in atoms, impressive physical insight and armed with a few simple rules, Dalton was able to construct a table of relative weights, which he first presented in 1803 at a talk to the Literary and Philosophical Society of Manchester. In 1805, this effort first appeared in print, with a systematic explanation of the method appearing in 1808 when Dalton published the first volume of his book A New System of Chemical Philosophy. Here, with hydrogen as his reference, he gave the following relative weights: hydrogen (H) 1; nitrogen (N) 5; carbon (C) 5.4; oxygen (O) 7; phosphorus (P) 9; sulfur (S) 13 and so on, including several elements and compounds.
Modern atomic theory began in 1808 when Dalton published A New System of Chemical Philosophy.
Proposed in 1811, Avogadro’s Hypothesis only began to gain acceptance in 1858 (two years after his death) thanks to Stanislao Cannizzaro.
Despite the various subatomic particles comprising an atom, only its outermost electrons are involved in chemical reactions.
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.
In 1789, Lavoisier published An Elementary Treatise on Chemistry where he describes 33 elements. The list begins with caloric and continues with light, oxygen, nitrogen and hydrogen.