Molecular graph

[4] For the purposes of enumeration of isomers, Cayley considered "diagrams" made of points labelled by atoms and connected by links into an assemblage.

If one continues to delete atoms connected by a single link further, one arrives at a mere kenogram, possibly empty.

In the early 18th century, Isaac Newton's notion of gravity had led to speculative ideas that atoms are held together by some kind of "gravitational force".

In particular, since 1758 Scottish chemist William Cullen in his lectures used what he called "affinity diagrams" to represent forces supposedly existing between pairs of molecules in a chemical reaction.

These and some other contemporary diagrams had no relation to chemical bonds: the latter notion was introduced only in the following century.