What did Cannizzaro say about the number of atoms in molecules of elementary gases?

Stanislao Cannizzaro argued that not all elementary gases consist of two atoms per molecule. In his 1858 pamphlet, Sunto di un corso di filosofia chimica fatto nella R. Università de Genova, he demonstrated that while most elementary gases are diatomic, some are monatomic and a few possess even more complex molecular structures.1

By reviving and championing Amedeo Avogadro's hypotheses, Cannizzaro provided the necessary evidence to resolve the mid-19th-century confusion regarding atomic weights and chemical formulas. His work established a consistent, unified system for determining these values, which remains the foundation of modern chemical notation.21