University of Illinois at Urbana-Champaign
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This paper contains the first appearance of modern chemical structural formulas. In 1864 Crum Brown began to draw pictures of molecules, in which he enclosed the symbols for atoms in circles (Dalton's notation), and used broken lines to connect the atoms together in a way that satisfied each atom's valence. He said: "an atom is represented by its usual symbol surrounded by a circle with as many lines proceeding from it as the atom contains equivalents. When equivalents mutually satisfy one another the two lines representing the equivalents are made continuous of one another." In other words, the two atoms form a bond. Crum Brown maintained that his structural formulas depicted the chemical, rather than physical, locations of the atoms with respect to one another. Later, however, chemists came to understand that the physical locations of atoms in space relative to one another could be determined experimentally, and that these physical locations were closely related to those given by Crum Brown's formulas.

Partington IV, pp552-553.

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