ABSTRACT
In chemistry, the first systematic correlations come from the conservation laws of mass and energy, followed by Dalton conception of structural matter. Cannizzaro revived Avogadro’s law in the First International Congress of Chemistry, which adopted the atomic weights of Gerhardt in Cannizzaro’s system after Dulong & Petit, based on Avogadro’s law. If the chemical elements are ordered by increasing atomic weight, then their chemical properties reveal a characteristic periodicity. Since the discovery of the atomic number, the true independent variable of Periodic Law, Periodic property is redefined: If the chemical elements are ordered by increasing atomic number, then their chemical properties reveal a characteristic periodicity. Periodicity is not general. However, if one accepts a natural order in the elements, its underlying law should be of phenomenic character. The problem is to find the underlying structure to the order.
