Born in a noble ancient family of Piedmont, Amedeo Avogadro was a brilliant student, he graduated in ecclesiastical law very young (20) and began to practice. However, soon after he dedicated himself to the study of physics and mathematics, his preferred sciences, and in 1809 he started teaching them (then called positive philosophy) at a liceo (high school) in Vercelli (where his family had some properties).
During this stay in Vercelli he wrote a concise note (memoria) in which he declared the hypothesis of what we now call Avogadro's law:
Avogadro's Law implies that the relationship occurring between the weights of same volumes of different gases (at the same temperature and pressure) corresponds to the relationship between respective molecular weights. Hence, relative molecular weights can be calculated from the weights of gases.
Avogadro developed this hypothesis after Joseph Louis Gay-Lussac had published in 1808 his law on volumes (and combining gases). The greatest difficulty Avogadro had to resolve was the huge confusion at that time regarding atoms and molecules – one of most important contributions of Avogadro's work was clearly distinguishing one from the other, admitting that simple particles too could be composed of molecules, and that these are composed of atoms. For instance, John Dalton didn't consider this possibility. Avogadro did not actually use the word "atom" as the words "atom" and "molecule" were used almost without difference. He considered that there were three kinds of "molecules," including an "elementary molecule" (our "atom"). Also, a keener attention was given to the definition of mass, as distinguished from weight.
In 1814 he published Mémoire sur les masses relatives des molécules des corps simples, ou densités présumées de leur gaz, et sur la constitution de quelques-uns de leur composés, pour servir de suite à l'Essai sur le même sujet, publié dans le Journal de Physique, juillet 1811 (), about gas densities.
In 1820 he became a professor of Turin's university; In 1821 he published another memoria, Nouvelles considérations sur la théorie des proportions déterminées dans les combinaisons, et sur la détermination des masses des molécules des corps and little after Mémoire sur la manière de ramener les composès organiques aux lois ordinaires des proportions déterminées.
With suspicious enthusiasm, he took part in political revolutionary movements of 1821 (against the king of Sardinia), so two years later he was removed from his position (or, as it was officially declared, the university was very glad to allow this interesting scientist to take a rest from heavy teaching duties, in order to be able to give a better attention to his researches). However, over time this political isolation was gradually reduced, since revolutionary ideas were receiving increasing attention fromSavoy kings, up to 1848 when Charles Albert granted a modern Constitution (Statuto Albertino). Well before this, following the increasing attention to his works, Avogadro had been recalled at Turin university in 1833, where he taught for another twenty years.
In 1841 he completed and published his work in Fisica dei corpi ponderabili, ossia Trattato della costituzione materiale de' corpi, 4 volumes.
Very little is known about his private life and his political activity; despite his unpleasant aspect (at least as depicted in the rare images found), he was known as a discreet tombeur de femmes although devoted to a sober life and a religious man. He had six children. Several historical studies would confirm that he had sponsored and helped some Sardinian plotters who were organising a revolution in that island, stopped at the very last moment by the concession of Charles Albert's statute. Some doubts however remain, considering the very little amount of evidence.
Avogadro held public posts in statistics, meteorology, and weights and measures (he introduced decimal metric system in Piedmont) and was a member of the Royal Superior Council on Public Instruction.
The scientific society didn't reserve a great attention at this theory, so Avogadro's hypothesis wasn't immediately accepted when announced. André-Marie Ampère too was able three years later to achieve the same result by another method (in his Sur la détermination des proportions dans lesquelles les corps se combinent d'après le nombre et la disposition respective des molécules dont leurs particules intégrantes sont composées), but the same indifferent regard was given to his theories as well.
Only with the studies by Gerhardt, Laurent and Williamson on organic chemistry, was it possible to demonstrate that Avogadro's law was indispensable to explain why same quantities of molecules, brought to a vapour state, have the same volume.
Unfortunately, in the performance of related experiments, some inorganic substances showed exceptions to the law. The matter was finally concluded by Stanislao Cannizzaro, as announced at Karlsruhe Congress (1860, four years after Avogadro's death), where he explained that these exceptions happened because of molecular dissociations which occurred at certain temperatures, and that Avogadro's law could determine not only molar masses, but as a consequence, atomic masses too.
Clausius, by his kinetic theory on gases was able to give another confirmation of Avogadro's law. Not long after, in his researches regarding dilute solutions (and the consequent discovery of analogies between the behaviour of solutions and gases), J. H. van 't Hoff added his final consensus for the triumph of the Italian scientist, who since then has been considered the founder of the atomic-molecular theory.