It came to him in a dream, Dmitri Mendeleev told a friend. He had worried at the problem of how to classify the elements for three sleepless days and nights. Exhausted, he fell into a deep sleep and the answer came.
Sadly, this may not be true. To begin with, Mendeleev – born in Siberia in 1834 – had been thinking about the problem since at least 1860, when he had heard Italian chemist Stanislao Cannizzaro establish the definitive atomic weight of the known elements at a scientific congress in Karlsruhe.
But it was also something numerous chemists had been working on since Antoine Lavoisier’s work on ‘simple substances’ in the 1780s. Many of Lavoisier’s substances were what we know as elements; some – charcoal, for instance – were not. The distinction, which seems obvious to us now, was an important one. “Atomic weight,” Mendeleev said, announcing his discovery, “belongs not to coal or diamond but carbon.”
By 1869, Mendeleev was professor of inorganic chemistry at St Petersburg and writing a textbook for his students called Principles of Chemistry. Having finished the first section, on the halogens, he paused to consider which group to cover next. How should the elements be organised?
If the answer did not come in a dream, then how did it come? It does seem to have happened quickly, on the morning of 1 March 1869 (17 February under the Julian calendar then in use in Russia). Mendeleev, a keen supporter of co-operative farming, had accepted an invitation to visit a cheese dairy. Instead of going, he turned over a letter from the dairy and began to scribble on the back the atomic weights of dissimilar elements. This was his Eureka moment. Many chemists had been looking at the problem through the prism of familial groups of elements; by comparing the unlike, Mendeleev was able to identify the general theory underlying all of them.
Here another possible myth intrudes. It is said Mendeleev made cards of all 63 known elements and played with them in a form of ‘chemical solitaire’ until he had worked out what we know as the Periodic Table. If true, the cards have disappeared – despite that fact that Mendeleev knew immediately he had done something extraordinary and kept all his notes – including the cheese-co-operative’s letter, complete with the print of his mug from breakfast.
Curiously, Mendeleev never revised the first part of his text book to acknowledge his discovery.
This piece first appeared in the March 2021 Months Past column in History Today.