Stanislao Cannizzaro writes in March 1858 a long letter to his colleague Sebastiano de Luca, published that same year as "Sunto di un corso di filosofia chimica" ("Outline of a Course in Chemical Philosophy"). In it he revives Avogadro's 1811 hypothesis, practically forgotten for almost half a century, and uses it to resolve the more-than-fifty-year confusion around atomic weights: he shows how vapor-density measurements allow unambiguous calculation of molecular weights from Avogadro's principle, and from there derive consistent atomic weights. The pamphlet, reissued in 1859, initially had little impact until Cannizzaro personally distributed it among attendees of the 1860 Karlsruhe Congress, where it ended up being the document that unlocked the consensus the conference itself had failed to reach.