The Law of Definite Proportions: Proust 1797

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This law is readily explained by Dalton's atomic theory. Suppose a compound is made of element A and element B, giving the formula AB. Since the weight of A is constant and the weight of B is constant, the A:B weight ratio will always be the same. Hence, the Law of Definite Proportions.


This law is sometimes called the Law of Constant Composition. In a modern textbook, it is:

A given chemical compound always contains the same proportion by mass of its constituent elements.

A different textbook says:

the relative amount of each element in a particular compound is always the same, regardless of preparation or source.

Here is what Proust said in 1797:

"I shall conclude by deducing from these experiments the principle I have established at the commencement od this memoir, viz. that iron like many other metals is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen. In this respect it does not differ from tin, mercury, and lead, and, in a word, almost every known combustible."

The above quote comes from a paper devoted to a complex substance known as Prussian Blue, discovered in 1704. Blueprints, developed in 1840, take their name from the Prussian Blue used to make them. The exact structure and composition of Prussian Blue has stimulated discussion up to modern times. For example, a reference book owed by the ChemTeam includes a reference to a 1973 article on Prussian Blue.

In the paper, Proust indicates that there are two oxides of iron, one with 27% oxygen and the other with 48% [his numbers]. Over the years to follow, he published a number of other examples. In 1804, he described the two oxides of copper thus:

  Copper Oxygen
Black oxide 100 25
Yellow or brown oxide 100 17-18

J.L. Partington says this in his "History of Chemistry":

"It has been said [by Kopp] that if Proust had calculated the weights of oxygen combining with a fixed weight of metal instead of the percentage composition of the two oxides, he would have discovered the law of multiple proportions. Proust actually did this; he generally gives the weights of oxygen combining with 100 of metal, but his analyses are too inaccurate to disclose any simple ratios . . . ."

For a number of years after 1797, Proust and his defenders engaged in a debate with C.L. Berthollet and his followers over this law. Although anti-Proust sentiment lingered in France for many years, by 1808 essentially all of the chemistry world had come to accept "Proust's Law" as correct.

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