1865 to 1929

Four answers to one rule, and the world's reply

Four benzene formulas on paper, then Lonsdale's diffraction spots resolving by calculation into a flat, regular ring On a sheet of paper, a pen draws four formulas for benzene by people who could not see molecules: Kekulé's ring of 1865 with alternating single and double bonds; Claus's 1866 hexagon with three bonds crossing the centre; Dewar's 1869 hexagon with one bond across the middle and two double bonds; and the prism of Claus, defended by Ladenburg in 1869, two triangles joined corner to corner. The paper fades. In 1929, X-rays through hexamethylbenzene crystals leave spots on a dark plate. From the spots, by calculation, a structure is worked out, and a ring of six carbons resolves, regular, each with its methyl group. The ring tilts edge-on and shows itself flat. The four drawings reappear faintly and fade; the regular ring remains. The animation repeats. C6H6 every carbon four bonds, one hydrogen
“In 1929 Kathleen Lonsdale's X-ray study of hexamethylbenzene found the ring flat and regular.” — Recognized, Not Devised The four formulas are drawn after Kekulé (1865), Claus (1866), Dewar (1869, listed but not endorsed) and the prism (Claus 1866, defended by Ladenburg 1869). The spots and the map are not Lonsdale's photograph or data: this page draws a density map from the spots of a model, a method Lonsdale used two years later. In 1929 she compared calculated intensities with measured ones. Either way the structure is worked out from a pattern, not seen.