Prior to the 1800s, the term compound had relatively little precise meaning. When used, it was often unclear as to whether one was referring to what scientists now call a mixture or to what they now know as a compound. During the nineteenth century, the debate as to the meaning of the word intensified, and it became one of the key questions in the young science of chemistry.
A critical aspect of this debate focused on the issue of constant composition. The issue was whether all compounds always had the same composition, or whether their composition could vary. The primary spokesman for the latter position was the French chemist Claude Louis Berthollet. Berthollet pointed to a considerable body of evidence that suggested a variable composition for compounds. For example, when some metals are heated, they form oxides that appear to have a regularly changing percentage composition. The longer they are heated, the higher the percentage of oxygen found in the oxide. Berthollet also mentioned alloys and amalgams as examples of substances with varying composition.
Berthollet's principal antagonist in this debate was his countryman Joseph Louis Proust. Proust argued that Dalton's atomic theory required that compounds have a constant composition, a position put forward by Dalton himself. Proust set out to counter each of the arguments set forth by Berthollet. In the case of metal oxides, for example, Proust was able to show that metals often form more than one oxide. As copper metal is heated, for example, it first forms copper(I) or cuprous oxide and then, copper(II) or cupric oxide. At any one time, then, an experimenter would be able to detect some mixture of the two oxides varying from pure copper(I) oxide to pure copper(II) oxide. However, each of the two oxides itself, Proust argued, has a set and constant composition.
Working in Proust's favor was an argument that nearly everyone was willing to acknowledge, namely that quantitative techniques had not yet been developed very highly in chemistry. Thus, it could be argued that what appeared to be variations in chemical composition were really nothing other than natural variability in results coming about as a result of imprecise techniques.
Proust remained puzzled by some of Berthollet's evidence, the problem of alloys and amalgams as an example. At the time, he had no way of knowing that such materials are not compounds but are in fact mixtures. These remaining problems notwithstanding, Proust's arguments eventually won the day and by the end of the century, the constant composition of compounds was universally accepted in chemistry.
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