 Danail Bonchev in his "Chemical Graph Theory" traces the origins of representation of chemical forces by diagrams which may be called "chemical graphs" to as early as the mid-18th century.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.