Lorenzo Romano Amedeo Carlo Avogadro

Lorenzo Romano Amedeo Carlo Avogadro

The Italian physicist and chemist Lorenzo Romano Amedeo Carlo Avogadro, Conte di Quaregna e di Cerreto (1776-1856), authored the hypothesis known as Avogadro's law, which ultimately clarified the foundations of molecular chemistry and physics.

Born in Turin on Aug. 9, 1776, Amedeo Avogadro came from an ancient legal family, whose name derived from the Latin de advocatis (concerning the law). He took a degree in philosophy in 1789, a baccalaureate in jurisprudence in 1792, and a doctorate in ecclesiastical law a few years later.

After several years of legal experience, Avogadro found his true avocation in the study of the physical sciences. Though largely self-taught, he achieved an extensive knowledge of the then-expanding studies of matter in the gaseous state. In 1809 he was appointed professor of physics in the Royal College at Vercelli. Up to that time his only scientific paper had concerned a topic in the new field of electricity.

His Great Memoir

In July 1811 Avogadro published his memoir in the Paris Journal de physique. He began by drawing attention to the discovery by the French chemist Joseph Louis Gay-Lussac that when gases combine they do so in simple integral proportions by volume. Gay-Lussac supplied the experimental evidence to generalize this property of volume ratios for all gases; that is, two volumes of ammonia (NH₃) are composed of one volume of nitrogen and three volumes of hydrogen, and so forth for many similar cases of simple, integral proportions.

On the basis of this type of evidence, Avogadro drew the logical conclusion that the number of "integrant molecules" in all gases is always the same for equal volumes. He also concluded that the ratios of the masses of the molecules are the same as those of the densities of the different gases at equal temperature and pressure and that the relative number of molecules in a given compound is given at once by the ratio of volumes of the gases that form it.

In a supplementary paper sent to the Journal de physique in 1814, Avogadro deduced the correct formulas for COCl₂, H₂S, and CO₂, and from postulating an analogy between carbon and silicon he asserted the correct composition of silica, SiO₂. From available data he calculated approximately correct atomic weights for carbon, chlorine, and sulfur. He contributed massively to an understanding of the properties and reactions of the new and "changerous" element fluorine. He published these and related findings in a four-volume work entitled Fisica de' corpi ponderabili, ossia trattato della constituzione generale de' corpi (1837-1841). This book influenced Michael Faraday's great career of discovery.

The simplicity and clarity of Avogadro's views, though cited by leading scientists, such as André Marie Amp'e, were not compelling to the majority of contemporary chemists. This lack of interest was due in part to the novelty of the atomic theories which had been presented to the world a few years before by John Dalton; furthermore, the methodological temper of the times, deeply experimentalistic and empirical, prevented careful consideration of a purely logical inference from chemical facts unsupported by masses of laboratory data.

Another confusing aspect of the Avogadro memoir was the use of the ambiguous term "molecule." Not only did this conflict with the vigorous Newtonian atomism of the English and French schools, but it implied a sequence of chemical reactions for which no decisive evidence was forthcoming. Dalton, for example, had postulated that water was formed by the simple addition of the element hydrogen to the element oxygen, or H + O → HO, whereas the correct process implicit in Avogadro's hypothesis was 2H₂+ O₂ (in the molecular form) → 2H₂O.

Other Activities

When the first Italian chair in mathematical physics was established at the University of Turin in 1820, Avogadro received the professorship. Two years later, because of the turmoil gripping the country, the chair was suppressed. Avogadro returned to his position in 1834 and held it until his retirement in 1850. He married Donna Felicita Mazzi, by whom he had six sons. Two sons rose to positions of distinction: Luigi, who became general of the Italian army, and Felici, who became president of the Court of Appeal.

Avogadro also served Italy as a competent and honest civil servant. He held positions in the National Bureau of Statistics, helped to establish a national meteorological service, and in 1848 became a member of the Superior Council on Public Instruction. Modest and retiring, he was indifferent to honors and scrupulously avoided those public struggles for priority which were a characteristic of Continental scientific society in the mid-19th century.

Some indication of the fundamental nature of Avogadro's law may be seen in the fact that when modern thermodynamic theory was established at the end of the 19th century, the great German scientist and eventual Nobel laureate Walter Nernst entitled his textbook Theoretical Chemistry from the Standpoint of Avogadro's Rule and Thermodynamics.

Further Reading

A discussion of Avogadro's life and work appears in J. R. Partington, A History of Chemistry, vol. 4 (1964). See also Sir William Augustus Tilden, Famous Chemists: The Men and Their Work (1921); Eduard Farber, The Evolution of Chemistry: A History of Its Ideas, Methods and Materials (1952; 2d ed. 1969); Henry M. Leicester and Herbert S. Klickstein, eds., A Source Book in Chemistry, 1400-1900 (1952); and Isaac Asimov, A Short History of Chemistry: An Introduction to the Ideas and Concepts of Chemistry (1965).

Additional Sources

Morselli, Mario, Amedeo Avogadro, a scientific biography, Dordrecht; Boston: D. Reidel Pub. Co.; Hingham, MA: Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 1984. □