Avogadros hypothesis (as it was known originally) was formulated in the same spirit of earlier empirical gas laws like Boyles law (1662), Charless law (1787)  and Gay-Lussacs law (1808). The hypothesis was first published by Amadeo Avogadro in 1811,  and it reconciled Dalton atomic theory with the "incompatible" idea of Joseph Louis Gay-Lussac that some gases were composite of different fundamental substances (molecules) in integer proportions. In 1814, independently from Avogadro, André-Marie Ampère published the same law with similar conclusions. As Ampère was more well known in France, the hypothesis was usually referred there as Ampère's hypothesis, and later also as Avogadro–Ampère hypothesis or even Ampère–Avogadro hypothesis.
Experimental studies carried out by Charles Frédéric Gerhardt and Auguste Laurent on organic chemistry demonstrated that Avogadros law explained why the same quantities of molecules in a gas have the same volume. Nevertheless, related experiments with some inorganic substances showed seeming exceptions to the law. This apparent contradiction was finally resolved by Stanislao Cannizzaro, as announced at Karlsruhe Congress in 1860, four years after Avogadros death. He explained that these exceptions were due to  molecular dissociations at certain temperatures, and that Avogadro's law determined not only molecular masses, but atomic masses as well.