  Aristotle (On Generation and Corruption, Bk 1, Ch 10) clearly identified what we would refer to as chemical change as a special category presenting problems peculiar to it. It differs from mere alteration, such as the browning of an autumn leaf, where an identifiable material substratum persists, and from generation and corruption, such as the transformation of an olive seed into a tree or the decay of a rose into a heap of dust, where no identifiable material substratum persists. The transformation of a mixture of copper and tin into bronze, an example of what Aristotle called combination, is intermediate between alteration and generation and corruption. Copper and tin do not persist as such in the bronze and to assume so would fail to make the appropriate distinction between a combination and a mixture. Nevertheless, there is some important sense in which the copper and tin are in the bronze because they are recoverable from it.  Aristotle had put his finger on a central problem in chemistry, the sense in which elements combine to form compounds and yet remain in the compounds as components of them. Aristotle did not use this terminology, of course, and it should be recognised that he and the scholastics that followed him had few examples of combination, as opposed to alteration and generation and corruption, to draw on.  Alloys, which provided them with their stock and just about only example, are not even compounds from a modern point of view. The importance of combination for Aristotelians lay in the philosophical challenge it posed.