On the Birth of Modern Chemistry: II—The Difficult Path of Avogadro’s Hypothesis Until Cannizzaro’s Full Acceptance Within the Atomic–Molecular Theory

This paper reviews the significant steps of the atomic–molecular theory, after Avogadro’s intimation of the equal volume/equal number of particles hypothesis until the final assertion embodied in Cannizzaro’s Sunto. Berzelius’s atomism, authoritatively present among chemists in the first decades of nineteenth century, is outlined. Applying volume theory, atomic weights were determined and later revised considering heat capacity experiments on solid elements and the law of isomorphism. The peculiar traits of Berzelius’s atomism are (a) the restricted validity of Avogadro’s hypothesis to only elementary gases, and (b) the opposition to the existence of elementary polyatomic molecules. Next, Dumas’ experiments on vapors are described, aimed at supporting Avogadro’s hypothesis, whose perplexing results were ingeniously resolved by Gaudin assuming that the elementary molecules may contain unequal numbers of component atoms. In the fifth decade of the century, Gerhardt and Laurent established molecular formulae with reference to standard volumes. Finally, at the end of the sixth decade, Cannizzaro published Sunto di un Corso di Filosofia Chimica, in which he fully acknowledges Avogadro’s hypothesis, together with all its implications, and describes how to arrive at molecular and atomic weights from gaseous densities. A brief account of the Karlsruhe congress is included, emphasizing the scientific personality of Cannizzaro.