Institute and Museum of History of Science, Florence, ITALY

Evangelista Torricelli 

3.4 The barometric experiment
schott.gif (36591 byte)

Gasparo Berti's experiment.
Engraving from Technica
by P. Schott ,
Herbipoli 1664.

The experiment on "quicksilver", carried out in Spring 1644, made Torricelli's name famous in Italy and transalpine Europe. The Italian scientists merit was, above all, to admit that the effective cause of the resistance presented by nature to the creation of a vacuum was probably due to the weight of air. This opinion, as it is known, was not shared by Galileo. The Florentine experiment was thus initially motivated by a desire to determine a possible relationship between the weight of the air and the resistence which encountered when attempting to produce a vacuum. A similar experiment had already been carried out in Rome, probably when Galileo was still alive, by Gasparo Berti, in the presence of the Jesuit fathers Niccol˜ Zucchi and Athanasius Kircher, but the results were only divulged in 1647. Berti had used water, so the tube had a length of around ten meters. Torricelli's original idea, and his technical contribution to the experiment, consisted in using mercury instead of water, an innovation which allowed the length of the tube to be reduced by a factor of thirteen. Even before carrying out the experiment, Torricelli asked himself if the column of mercury would leave an empty space behind when it descended to a position of equilibrium with the column of stmospheric air outside the tube. Torricelli's experiment provoked enormous interest, particularly in France and Poland. The discussions were not always focussed on the technical aspects of the experiment, or the scientific conclusions which it was possible to draw from it, but instead raised yet again the polemic between "ancients" and "moderns". The partial drop of the level of the mercury in the tube produced an apparently empty space at the top of the tube, placing one of the basic principles of Aristotelian physics in jeopardy. The Jesuits fought with conviction to defend the impossibility of the vacuum. One might reasonably wonder about the extent of Torricelli's engagement with the debate. The answer is simple: he did not participate at all. In two letters addressed on the 11 and 28 June 1644 to Michelangelo Ricci, the mathematician of the Grand Duke described the experiment, but took no position in the philosophical debate raised by the apparent vacuum. He simply observed, in the letter of 11 June, that "many have said that the vacuum does not exist, others that it can exist but only with difficulty and against the repugnance of Nature". Additionally, Torricelli did not consider the experiment to have succeeded because the height of the column of mercury which had to balance the weight of the air was caused to vary "by the heat and the cold". In any case, one can reach the conclusion that the value of the weight of the air proposed by the "ancients" was completely erroneous. Another very important result was that the force that prevented the mercury from falling was not inside the tube; Torricelli, in the same letter, proposed the hypothesis that this force was external, and was due to the "heaviness of the air". The two letters to Ricci are the only documents written by Torricelli himself on the subject. It might be hypothesized that this silence was due to his unhappiness at the intervention of theologians in the debate. A phrase of Ricci's, in a letter sent to Torricelli on 18 June 1644, provides some support for this hypothesis: "I estimate that you will unfortunately be too nauseated by the temerarious opinion of these Theologians, and by their constant habit of mixing up things of God with natural questions, where they should instead be treated with greater respect and reverence" The motivations behind the participation in the debate by eminent dignitaries of the Roman church are complex and still not fully clarified at the present time. In any case, one can observe that for the defenders of the Thomist-Aristotelian tradition, the existence of the vacuum, which was generally associated with that of atoms, recalled the philosophy of Leucippus, Democritus and Epicurus, whose followers were considered closer to heresy than to Catholic orthodoxy on certain issues. It is not to be excluded that atomism was combatted because of the difficulties which it would generate for a reliable interpretation of the "dogma of transubstantiation".


Drawing from Torricelli's
letter of June 11 1664 to M.
Ricci. It is published in
Opere dei Discepoli di
, Florence 1975.

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