The law of definite proportions states that a chemical compound always contains the same proportion of elements by mass. That is to say, changing the scale of a sample of a compound does not alter its proportionate composition.
The law is often attributed to John Dalton, but it was inspired by a French chemist named Joseph Proust, whose experimental work between 1798 and 1804 led him to the conclusions that formed the law. Dalton's related Atomic Theory built on this foundation and the two became heavily interrelated in theory and in the scientific community's lexicons both historical and procedural.
The related law of constant composition states that any sample of a chemical compound will have the same elemental proportions by mass, proving total internal coherence for the sample and compound. These principles are of the utmost importance in understanding the laws of physics which are descriptions, in essential nature, of the way that chemicals interact with one another in the phenomenal world.
These laws are often referred to as Proust's Laws but are very often mentioned in connection with Dalton. Dalton proved that atoms have a consistent weight and his work was directly rooted in Proust's, depending upon its conclusions both for its thesis and for the many experiments needed to establish that thesis as a valid theoretical framework.