Quantum refert in quae tempora vel optimi cujusque virtus incidet. (From an architrave on the Palatine.) The history of the development of scientific thought offers several examples of unconventional, perplexing and untimely conjectures advanced by obscure men. The molecular hypothesis set forth by Amedeo Avogadro in 1811 is one of these. It was little noticed when proposed and it soon fell into oblivion. Although numerous attempts have been made in the past one hundred years to identify the main reasons for the neglect and the indifference which surrounded the molecular hypothesis, very...