"To lead my students to the same conviction I have, I wanted to place them on the same path across which I arrived to it, that is the chemical theories' historical examination" Cannizzaro, 1858

Avogadro had his first recognition after 1807, when he presented the first paper to the Science Academy of Turin where he describes how to molecules of a idioelectrical body (not wire) as the air situated between two surfaces with opposite charges happened that phenomenon later called from Faraday polarization of dielectrics, the study of which will be resumed with a century of distance by Van der Waals in the development of the intermolecular forces theory. The subject had gained attention after the invention of the Volta'spile and the dispute between him and Galvani on the nature of the fluid accountable for elecrical phenomena. In the 1809's Idées sur l'acidité et l'alcalinité, on the strength of the idea supported by Humphry Davy (with the experiment of sodium electrical separation), that the chemical affinity was determined by electrical forces, he elaborates a new electrical conception of substances' alkalinity and acidity, since the year dot considered chemical reactivity's central qualities and since a short time reproposed as such from Berthollet. Furthermore Avogadro claimed that oxygenity (acidity) and basicityhadn't to be considered dependent from the simple bodies' composition with the oxygen but from the electrical relative state of the same bodies (it had just discovered compounds that Gay-Lussac'll call hydracids, which had upset lavoisierian models) and that these ones were compound's relative properties, bringing forward of a century the Brönsted and Lowry's modern definition of acid and base. So each chemical reaction could be traced back to an acid-base antagonism that is an electrical dualism.

In the 14 July 1811's editionof the De Lamétherie's Journal de Physique, de Chimie et d'Histoire naturelle Avogadro spreads a previous memory considered a classic of the modern science, the

"Essai d'une manière de déterminer les masses relatives des molecules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons

[Essay on a manner of determining the relative masses of the elementary molecules of bodies, and the proportions in which they enter into these compounds]  

The essai's topic is the pneumatic chemistry (gas's one), main eighteenth century chemistry's branch, basis of the atomic theory worded by John Dalton in the 1808's New system of chemical philosophy, they turned their attention to which form 1809 when Joseph Louis Gay-Lussac published the Mémoire sur la combinaison des substances gazeuses, les unes avec les autres, where, working indipendently by Dalton, he studied the thermic expansion of some gasses (air, hydrogen, oxygen, ammonia) in a thermic interval from 0 to 100 °C: these ones expanded likewise with corresponding thermic variations (Gay-Lussac's law). 

Despite the Proust's law of definite proportions of 1798 afterwards have seemed particular case of the daltonian law of multiple proportions, Dalton himself regarded the first didn't provide for experimental differences between combination ratios between volumes for example of hydrogen and chlorine (1:1) and of hydrogen and oxygen (2:1). Avogadro pinpointed the combination laws' theoretical explanation in the invisibles integral, constituent and  elementary molecules and in their interaction with the caloric, the fluid that combining with the ordinary matter was believed to have the capacity to bring it to the gassy state. Thus intent attained by him was to establish qualitatively (densities method) reagents' molecular mass, "since molecular masses' ratios are then the same than gasses densities' ones, with equal pression and temperature, and the molecules' relative number in a combination is directly given by the volumes' ratio of the formed gasses". Actually one thought that (Avogadro's law) "equal volumes of gasses, at the same temperature and pression, contain the same number of molecules”  integral made up by several elementary molecules: even if for the first time he distinguished atoms from molecules, it must be specified that Avogadro in his writings continues to think back to a molecularist language following the French chemical tradition from phlogistic theories to Berthollet 'n' Fourcroy and in his works you cannot find the term atom. Not secondary worth of the Essai was finally the acknowledge that also elements (nitrogen, oxygen, hydrogen, chlorine) could be formed by molecules (which Dalton hadn't done).

During those years Jöns Jacob Berzelius tried out Dalton atomic theory's application to the traditional electrochemistry distinguishing in every compound two opposites electrical poles, which dissociated in solution. In indifferent (neutral) substances poles were balanced, while in load ones a pole was prevalent. So he invented an electrochemical system, in which only the oxygen  was absolutely electronegative, whilst other elements, or radical, were disposed in a decreasing scale according to elecronegativity. 

Avogadro, who still will recognizes in the Estratto di diverse Memorie sulle affinità dé corpi pel calorico in 1824 the derivation of this theory by his ideas, in the Réflexions sur la théorie électro-chimique de M. Berzelius in 1813 rejects the idea that oxygen is absolutely electronegative and thinks there aren't indifferent substances. In the Réflexions he'll rebuke further on to the Swede for not having followed his concepts of oxygenity and basicity. In another work on this topic, published in 1821 with the title Nouvelles considérations sur la théorie des proportions déterminées dans les combinaisons, et sur la détermination des masses des molécules des corps, he'll add formulas and relative molecular weights of inorganic compounds like oxygenized chlorine and phosphorus compounds, and organic ones like ethyl alcohol, ether and urea. Both here and in the immediately successive Mémoire sur la manière de ramener les composès organiques aux lois ordinaires des proportions déterminées he confutes besides the ontological discrimination (Berzelius) between inorganic and organic compounds, aiming at supporting vitalist currents in chemisty. 

In 1814 was born the 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.

In that paper he spins off the previous hypothesis and adds on to water, nitrogen and carbon oxides, ammonia and hydrochloric acid ones obtained in 1811 many other formulas among which phosphine, sulphuretted hydrogen, carbon sulphide, silicon oxide. In the same year Ampère broadasted a memory in the form of letter  entitled Sur la détermination des proportions dans les quelles les corps se combinent d'après le nombre et la disposition respective des molécules dont leurs particules intégrantes sont composées in which he exposes results similar to that ones contained in the Essai, but with different grounding and goals: for Ampère the molecule is a dot-like centre of attractive and repulsive forces, while the particle represents a whole of molecules geometrically disposed in the three-dimensional space, since he had the purpose to give 

the chemical combination a strict geometric (crystallographic) wording.

From 1816 to 1825 he published a sequence of memories about the combination between various bodies and the caloric and about the possibility to fix their specific calorics, that will last until 1835, despite of the discovery by Dulong and Petit of the relation between specific heat and atomic weight of simple bodies. Besides some other important works of this period will have as topic atomic volumes and electricity.

The merit of the clear distinction between atoms and molecules is dued in the opinion of Mendeléeff to Stanislao Cannizzaro at the international congress of Karlsruhe (3-5 September 1860, organized by Kekulé, who had precisely as end the search of "more precise definitions of the concept of atom, molecule, equivalent, atomicity, alkalinity, etc.: ... the true equivalents of bodies and their formulas; start of a plan for a rational nomenclature"), with which it end off the epic of the new atomist theory's achievement: "We decide to give a different sense to molecules and atoms, considering as molecule the quantity of substance which enters in a reaction and which causes the physical property, and intending as atom the smallest quantity of substance included in a molecule".

The discrimination had already been suggested in 1858 in his Lezione sulla teoria atomica and Sunto di un corso di filosofia chimica (key text of the congress), where he asserts to be debtor of it toward Gaudin, Ampère's pupil. Whereas this one will adopt in 1835 the new terminology, Avogadro, still in 1838 in his summa Fisica dé corpi ponderabili (the first italian technical physics textbook) will stay on strongly fond of his molecularist jargon nevertheless reporting sometimes the term atom as technical one in someone else's studies (for example in the treatment of Dumas's research on sulphur's and phosphorus's vapour density).

Not secondary merit of Cannizzaro (and of the congress) has been finally the historical confirmation of the Essai's hypothesis: