Mathematical chemistry is often thought to be a 20 th -century sub- discipline of chemistry, but in this paper we discuss several early chemical ide- as and some landmarks of chemistry as instances of the mathematical way of thinking; many of them before 1900. By the mathematical way of thinking, we follow Weyl's description of it in terms of functional thinking, i.e. setting up variables, symbolizing them, and seeking for functions relating them. The cas- es we discuss are Plato's triangles, Geoffroy's affinity table, Lavoisier's classifi- cation of substances and their relationships, Mendeleev's periodic table, Cay- ley's enumeration of alkanes, Sylvester's association of algebra and chemistry, and Wiener's relationship between molecular structure and boiling points. These examples show that mathematical chemistry has much more than a cen- tury of history.