Aug 092018

Today is the birthday (1776) of Amedeo Carlo Avogadro, count of Quaregna and Cerreto a scientist, most noted for his contribution to molecular theory now known as Avogadro’s law, which states that equal volumes of gases under the same conditions of temperature and pressure will contain equal numbers of molecules. In tribute to him, the number of elementary entities (atoms, molecules, ions or other particles) in 1 mole of a substance, 6.022140857×1023, is known as the Avogadro constant, one of the seven SI base units .

Avogadro was born in Turin to a noble family of Piedmont-Sardinia in the year 1776. He graduated in ecclesiastical law at the late age of 20 and began to practice. Soon afterwards, he dedicated himself to physics and mathematics, and in 1809 started teaching them at a liceo in Vercelli, where his family lived and had some property. In 1811, he published “Essai d’une manière de déterminer les masses relatives des molécules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons” (“Essay on Determining the Relative Masses of the Elementary Molecules of Bodies and the Proportions by Which They Enter These Combinations”), which contains Avogadro’s hypothesis concerning gases.

In 1820, he became a professor of physics at the University of Turin. Turin was now the capital of the restored Savoyard Kingdom of Sardinia under Victor Emmanuel I. Avogadro was active in the Risorgimento activities of March 1821. As a result, he lost his chair in 1823 (or, as the university officially declared, it was “very glad to allow this interesting scientist to take a rest from heavy teaching duties, in order to be able to give better attention to his researches”). Eventually, Charles Albert, king of Sardinia, granted a Constitution (Statuto Albertino) in 1848. Well before this, Avogadro had been recalled to the university in Turin in 1833, where he taught for another 20 years.

Little is known about Avogadro’s private life, which appears to have been sober and religious. He married Felicita Mazzé and had as many as eight children. Avogadro held posts dealing with statistics, meteorology, and weights and measures. He introduced the metric system into Piedmont and was a member of the Royal Superior Council on Public Instruction. He died on 9 July 1856, a month shy of his 80th birthday.

In honor of Avogadro’s contributions to molecular theory, the number of molecules in one mole was named Avogadro’s number, also NA, or Avogadro’s constant. It is approximately 6.0221409×1023. Avogadro’s number is used to compute the results of chemical reactions. It allows chemists to determine amounts of substances produced in a given reaction to a high degree of accuracy. In actual fact, Johann Josef Loschmidt first calculated the value of Avogadro’s number, often referred to as the Loschmidt number in German-speaking countries (Loschmidt constant now has another meaning).

Avogadro’s Law states that the relationship between the masses of the same volume of all gases (at the same temperature and pressure) corresponds to the relationship between their respective molecular weights. Hence, the relative molecular mass of a gas can be calculated from the mass of sample of known volume. Avogadro developed this hypothesis after Joseph Louis Gay-Lussac had published in 1808 his law on volumes (and combining gases). The greatest problem Avogadro had to resolve was the confusion at that time regarding atoms and molecules. One of his most important contributions was clearly distinguishing one from the other, stating that gases are composed of molecules, and these molecules are composed of atoms. For instance, John Dalton did not consider this possibility. Avogadro did not actually use the word “atom” as the words “atom” and “molecule” were used almost without difference. He believed that there were three kinds of “molecules,” including an “elementary molecule” (our “atom”). Also, more attention was given to the definition of mass, as distinguished from weight.

In 1815, he published “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 composes” (“Note on the relative masses of elementary molecules, or suggested densities of their gases, and on the constituents of some of their compounds), as a follow-up to his essay on the same subject, published in 1811. In 1821 he published another paper, “Nouvelles considérations sur la théorie des proportions déterminées dans les combinaisons, et sur la détermination des masses des molécules des corps” (New considerations on the theory of proportions determined in combinations, and on determination of the masses of atoms) and shortly afterwards, “Mémoire sur la manière de ramener les composès organiques aux lois ordinaires des proportions déterminées” (Note on the manner of finding the organic composition by the ordinary laws of determined proportions”).

The scientific community did not give great attention to his theory, so Avogadro’s hypothesis was not immediately accepted. André-Marie Ampère published a very similar theory three years later, but the same indifference was shown to his theory as well. Only through studies by Charles Frédéric Gerhardt and Auguste Laurent on organic chemistry was it possible to demonstrate that Avogadro’s law explained why the same quantities of molecules in a gas have the same volume. Unfortunately, related experiments with some inorganic substances showed seeming exceptions to the law. This was finally resolved by Stanislao Cannizzaro, as announced at Karlsruhe Congress in 1860, four years after Avogadro’s death. He explained that these exceptions were due to molecular dissociations at certain temperatures, and that Avogadro’s law determined not only molecular masses, but atomic masses as well. Now, Avogadro is hailed as one of the founders of atomic-molecular theory.

Turin, and Piedmont in general, is loaded with culinary specialties. Anyone who knows Turin knows that you cannot visit the city without sampling its chocolate: in confections or as a drink. I spent the weekend before Lent 2 years ago in Turin and bathed in chocolate in between Carnevale events. I also had some stupendous dishes. Turin salame is famous, as are bollito misto, agnolotti, and flan di verdure (sometimes served with fresh anchovies). Perhaps the local favorite is bagna caoda, which has been around for centuries, and is typically served as part of Christmas Eve dinner (or any time you want). It is a flavorful sauce served at the table like fondue (over a low flame), and typically diners dip in cardoons which they then eat with bread, allowing some of the sauce to drip on the bread. Cardoons are the blanched stalks of a species of thistle that is hard to find outside of Mediterranean countries. You can use celery, asparagus, or a mix of vegetables as the Piedmontese often do.

Bagna Caoda


¾ cup extra virgin olive oil
6 tbsp unsalted butter at room temperature
12 fresh anchovy fillets
6 large garlic cloves, peeled and chopped
salt and pepper
cardoons (or assorted vegetables)
1 loaf crusty Italian bread, cut in thick slices


Place the olive oil, butter, anchovies and garlic in a food processor and blend until smooth. Transfer the oil mixture to heavy medium saucepan. Cook over low heat for 15 minutes, stirring occasionally.  Season to taste with salt and pepper.

Pour the sauce into fondue pot or any container you can set over a flame at the table. Set the pot over a table burner at the table. Serve the vegetables and bread on large platters so that diners can help themselves.