  Dalton was able to take for granted assumptions that had become central to chemistry since the work of Lavoisier. Chemical compounds were understood as arising through the combination of chemical elements, substances that cannot be broken down into something simpler by chemical means. The weight of each element was understood to be preserved in chemical reactions. By the time Dalton (1808) made his first contributions to chemistry the law of constant composition of compounds could be added to this. Proust had done much to substantiate experimentally the claim that the relative weights of elements making up a chemical compound remain constant independent of its mode of preparation, its temperature and its state.