Chemistry, Physics, Combination

Aristotle is generally interpreted as having devoted little attention to Chemistry. However, though the relevant passages are not nearly as prominent as are his works on Physics, Psychology, Astronomy, and Biology, in On Generation and Corruption, he does examine a pattern that is characteristic of Chemistic events--combination, which entails simultaneous reciprocity.  In other words, on that analysis, if A and B 'combine', they simultaneously act upon one another.  But, then, what remains unclear is as which of the four Causes of Physics such combination can be classified, i. e. simultaneous reciprocity seems to be a characteristic of none of them.  In the absence of further clarification, Chemistry seems to elude a reduction to his Physics.