Who is the law of definite proportion or constant composition usually attributed to?

The law of definite proportions, also known as the law of constant composition, states that a chemical compound always contains the same proportion by mass of its constituent elements, regardless of the source or quantity of the compound. This principle is fundamentally important in chemistry as it supports the concept that compounds are fixed entities composed of specific ratios of elements.

The law is primarily attributed to Joseph Proust, who, through his experiments in the late 18th century, demonstrated that substances are composed of a defined set of elements that combine in fixed ratios. His work challenged the then-existing views on the variability of composition in mixtures, leading to a better understanding of chemical compounds and their properties.

In contrast, Avogadro is known for his hypothesis about the volumes of gases, Dalton is known for his atomic theory, and Boyle is recognized for Boyle's law regarding gases. While each contributed significantly to the field of chemistry, it is Proust who is specifically credited with the law of definite proportions. This distinction highlights the significance of consistent chemical composition, a foundational concept in understanding the behavior of elements and compounds.