A law with older roots than the credit suggests
The claim is that in any chemical reaction confined to a closed system, the total mass present at the end equals the total mass present at the start, however much the substances themselves have changed. This was not entirely new when Lavoisier took it up: Jean Rey had already demonstrated something close to it in a chemical context as early as 1630, and Mikhail Lomonosov had formally articulated the principle by the 1750s while separately challenging the phlogiston theory then dominant in chemistry. What Lavoisier added was not the idea itself but a standard of experimental proof rigorous enough to make the idea unavoidable.
Weighing instead of arguing
His method was to weigh reactants and products of a chemical reaction while it took place inside a sealed glass vessel, so that no gas produced or consumed during the reaction could escape unmeasured and quietly distort the final tally. This closed-vessel weighing was paired with an exactness of measurement, reportedly to five or eight decimal places, that some contemporaries considered excessive for the purpose; the historical value of that precision is exactly that it left much less room for an inconvenient result to be explained away or dismissed as ordinary measurement error.
What the balanced scale proved
By the mid-1770s this approach had produced a clear result: although the substances in a reaction visibly changed state or form, the total mass measured at the end matched the total measured at the beginning, reaction after reaction. That finding, demonstrated repeatedly and with unusual care, is what let Lavoisier argue that combustion involved the combination of a material with oxygen rather than the release of some separate weightless substance, since the balanced mass left no room for anything at all to have escaped the closed system unaccounted for during the process.
The theory that didn’t survive the numbers
This is exactly what phlogiston theory could not survive. Phlogiston was proposed as a substance released from burning material, but Lavoisier’s sealed, weighed experiments left no unexplained mass loss for a departing phlogiston to account for, and his identification of oxygen supplied a more complete alternative explanation for the same reactions. His 1783 memoir, Réflexions sur le phlogistique, made the case against the old theory directly, and by the time his 1789 textbook, Traité élémentaire de chimie, set out chemistry’s new foundations, conservation of mass and the rejection of phlogiston were presented as settled.
The limit later physics found
The law’s modern limits come from a different direction entirely. Einstein’s special relativity introduced mass-energy equivalence, meaning mass and energy can convert into one another, and the Cockcroft-Walton experiment in 1932 provided the first direct confirmation of measurable mass loss during a nuclear transmutation. Conservation of mass as Lavoisier established it still holds to extremely high precision in ordinary chemical reactions, where the energies involved are far too small for any such conversion to be detectable, but it is not, strictly, an unconditional law of the universe once nuclear-scale processes are in play.
A method worth more attention than the man’s ending
This is worth the time because it separates two things popular memory tends to fuse: a genuinely durable scientific result, still taught and used in chemistry classrooms exactly as Lavoisier established it, and the dramatic ending of the man who established it, guillotined during the French Revolution over his separate role in tax collection. The chemistry does not depend on the biography, and readers who come away remembering only the execution have missed the more useful part — a demonstration of how a disciplined habit of weighing things, rather than any single flash of insight, is what actually ended a wrong theory that had persisted for generations.