Proust's best known work was derived from a controversy with C.L. Berthollet. Berthollet did not believe that substances always combine in constant and definite proportions as Proust did. Proust eventually was able to prove Berthollet wrong in 1799 and published his own hypothesis. law of definite proportionslaw of definite proportions, which is sometimes also known as Proust's Law chemical compound always contains exactly the same proportion of elements by mass.

He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness (sometimes referred to as Daltonism, in his honour). In chemistry, the law of multiple proportions is one of the basic laws of stoichiometry used to establish the atomic theory, alongside the law of conservation of mass (matter) and the law of definite proportions. It is sometimes called Dalton's Law after its discoverer, the English chemist John Dalton, who published it in the first part of the first volume of his "New System of Chemical Philosophy" (1808). The statement of the law is: If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers. In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances. This paper was published in 1805, but he did not discuss there exactly how he obtained these figures.

It is generally accepted that Lavoisier's great accomplishments in chemistry largely stem from the fact that he changed the science from a qualitative to a quantitative one. Lavoisier is most noted for his discovery of the role oxygen plays in combustion. He recognized and named oxygen (1778) and hydrogen (1783) At the height of the French Revolution, he was accused by Jean- Paul Marat of selling adulterated tobacco and of other crimes, and was eventually guillotined Lavoisier's experiments supported the law of conservation of mass. In France it is taught as Lavoisier's Law and is paraphrased from a statement in his "Traité Élémentaire de Chimie" to "Rien ne se perd, rien ne se crée, tout se transforme." ("Nothing is lost, nothing is created, everything is transformed."). Mikhail Lomonosov (1711–1765) had previously expressed similar ideas in 1748 and proved them in experiments; others whose ideas pre-date the work of Lavoisier include Jean Rey (1583–1645), Joseph Black (1728–1799), and Henry Cavendish (1731–1810). His Traité élémentaire de chimie (Elementary Treatise on Chemistry), published in This work represents the synthesis of Lavoisier's contribution to chemistry and can be considered the first modern textbook on the subject.

Lavoisier's Laboratory, Musée des Arts et Métiers, Paris

He is most noted for his contributions to molecular theory, including what is known as Avogadro's law. In tribute to him, the number of elementary entities (atoms, molecules, ions or other particles) in 1 mole of a substance, (30)×10 23, is known as the Avogadro constant For an ideal gas, the volume and amount (moles) of the gas are directly proportional if the temperature and pressure are constant. In 1811, he published an article with the title Essai d'une manière de déterminer les masses relatives des molécules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons ("Essay on Determining the Relative Masses of the Elementary Molecules of Bodies and the Proportions by Which They Enter These Combinations"), which contains Avogadro's hypothesis

Avogadro constant The French physicist Jean Perrin in 1909 proposed naming the constant in honor of Avogadro. Perrin won the 1926 Nobel Prize in Physics, largely for his work in determining the Avogadro constant by several different methods. The value of the Avogadro constant was first indicated by Johann Josef Loschmidt who in 1865 estimated the average diameter of the molecules in air by a method that is equivalent to calculating the number of particles in a given volume of gas.