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In 1858, two years after Amedeo Avogadro's death, his fellow Italian Stanislao Cannizzaro (1826–1910) outlined a course in theoretical chemistry for students at the University of Genoa—where he had to teach without benefit of a laboratory. He used Avogadro's hypothesis as a pathway out of the confusion rampant among chemists about atomic weights and the fundamental structure of chemical compounds.

By all accounts Cannizzaro was much clearer in his explanations than Avogadro, and as an organic chemist he also showed how Avogadro's ideas could be applied to this branch of chemistry. In 1860 the first international chemical congress was held in Karlsruhe, Germany, to settle some of the contemporary chemical disputes—how to define molecule and atom, what chemical nomenclature to use, how to determine atomic weights, and so on. After much discussion the chemists agreed to return home to decide for themselves how to proceed. However, many participants carried away a handout, a printed version of Cannizzaro's outline, that seemed convincing upon later reading.

At this time Cannizzaro was in the midst of eventful chemical and political careers. He was born in Palermo, Sicily, where his father was a magistrate and the minister of police, and he later attended medical school there, which kindled an interest in chemistry. Despite his family's connections to the royal court in Naples, he joined the antimonarchical 1848 revolution in Sicily. When it failed, he fled to Paris, where he resumed his chemical studies. After returning to Italy, he held academic appointments in Alessandria, where he worked out the "Cannizzaro reaction"—the self-oxidation and self-reduction of aldehydes—and Genoa, where he expounded Avogadro's hypothesis. He next supported Giuseppe Garibaldi's Sicilian revolt of 1860 and took part in the new government centered in Palermo. During this time he expanded the program of chemical studies at the university there. Upon Italian unification in 1871 he moved to Rome, where he continued his roles as a public figure and as a chemical scientist and educator.