Garzoni, Leonardo S. J.

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(b. Venice, Italy, 1543, d. Venice, 10 March 1592),


The little data researchers have on Garzoni’s life are the brief notices registered on official documents of the Society of Jesus. From these sources it is known that Garzoni was born into a patrician family and that he began his philosophical studies before 1565. Around 1565, he joined a congregation near the Jesuits’ College in Brescia and entered the Society of Jesus in 1567 or 1568. In 1568, he lectured in logic in Parma, and in 1573, he was a third-year student in theology in Padua. On 9 June 1579, he took his four vows in Brescia and from 1579 lived, as a confessor, in Venice. After a stay in Verona (around 1588) he came back to Venice, where he died.

Garzoni’s only extant work, the Due trattati sopra la natura, e le qualità della calamita, is the first known example of a modern treatment of magnetic phenomena. Written in years near 1580 and never published, the treatise was widely disseminated immediately after Garzoni’s death. In particular, Garzoni is referred to as an expert in magnetism by Niccolò Cabeo, whose Philosophia Magnetica (1629) is simply a readjustment of Garzoni’s work. More interestingly, Garzoni’s treatise was known also to Giovan Battista Della Porta and William Gilbert. Even if the Jesuit is never mentioned, both Della Porta’s Magia Naturalis(1589) and Gilbert’s De Magnete (1600) show a heavy dependence on Garzoni’s treatise. In the case of Della Porta, there was blatant plagiarism, as has already been remarked upon by Niccolò Cabeo (Philosophia Magnetica, Praefatio ad lectorem) and Niccolò Zucchi (Philosophia magnetica…, fols. 62v–63r).

The Due trattati sopra la natura, e le qualità della calamita is composed of two parts, or treatises. In the first treatise, consisting of seventeen chapters, Garzoni works out the details of his theory of magnetism. The second treatise is divided into two sections. The first section contains a description of a number of experiments, presented as ninety conclusions or doubts, and thirty-nine corollaries. The second section contains the theoretical interpretation of the experiments.

After a few short remarks about the crucial role of experiments in a scientific investigation, Garzoni outlines the aims and contents of the work: The author’s goal is to explain the two principal magnetic effects displayed by the lodestone, namely, its tendency to the poles and its interaction with other lodestones, or with iron. In fact, Garzoni says, the cause of every magnetic effect can be understood once the common cause of both the motion that aligns the stone in the direction of the poles and the motion that aligns two stones relative to each other is understood. Garzoni’s theory can be summarized as follows. First, the author ascertains that the motion toward the poles is a natural one, ascribes it to an internal mover—namely, the substantial form of the lodestone— and says that, in order to carry out the motion, the form needs an appropriate instrument, which he calls the “quality of two faces.” The lodestone naturally possesses the quality, or verticity, while iron can acquire it from the stone, thus becoming magnetized. Iron naturally possesses a similar quality, or “quality of one face,” by which means it is disposed to receive verticity from a lodestone. Once magnetized, iron behaves exactly like a lodestone. Of particular interest are the chapters devoted to the description of the way verticity moves the lodestone and the way it alters surrounding bodies, virtually propagating itself outside the stone, in the so-called sphere of activity. To explain the different ways in which the quality exists in the stone, in the medium and in the iron, Garzoni draws a comparison with light, in line with the medieval tradition of the perspectivists. Other interesting features arise out of the explanation of the double nature of magnetic quality, and out of the problem of the location of the (celestial) magnetic poles.

In the second treatise, entirely devoted to the experiment, Garzoni lists a number of magnetic phenomena. The first experimental result listed is the one showing the proper alignment of the lodestone to the poles. Some observations about the interaction between lodestones follow, and Garzoni then considers the interaction between the lodestone and iron. A long series of experiments devoted to the transmission of the magnetic virtue is discussed. The observations are very carefully managed. Garzoni then considers the diffusion of the magnetic virtue inside the stone, outside it, and inside iron. These results are obtained by magnetizing bodies of different shapes and sizes. In particular, Garzoni considers the behavior of magnetized iron dust. The behavior of iron placed in the sphere of action of one or more lodestones is subsequently investigated. In this case, the experiments are also managed with bodies of different shape and size.

In subsequent experiments, Garzoni studies the external diffusion of magnetic virtue by displacing a magnetic needle within the sphere of action. At every point, the direction of the needle gives the direction of the magnetic virtue: In fact, the virtue of the needle is far lower than the virtue of the lodestone, so that the action of the needle on the stone is negligible. The configuration that Garzoni obtains coincides with the one theorized in the first treatise, where the accompanying diagrams look remarkably modern in their outline of the lines of propagation of verticity. Garzoni then studies the behavior of two magnetized needles. Given that, in this case, the action is not negligible, the effects are very different from those recorded before, unless one of the needles is considerably smaller (or weaker) than the other. In this case, one observes the same results as when using a needle and a lodestone. Then Garzoni investigates the action of non-magnetized iron, the properties of the quality of one face, and the alteration of the quality of one and two faces. Finally, he quickly mentions spontaneous magnetization. The first section of the second treatise ends with two marginal observations about the loss of verticity.



Trattati della calamita, a cura di M. Ugaglia. Milano: Franco Angeli, 2005. Edition and analysis of the only extant manuscript (Milan, Biblioteca Ambrosiana, S 82 SUP) of Garzoni’s treatise.


Baldini, Ugo. Legem impone subactis: Studi su filosofia e scienza dei gesuiti in Italia 1540–1632. Roma: Bulzoni, 1992. The book contains the first mention of the surviving copy of Garzoni’s treatise, together with a discussion of Garzoni’s role within the Society of Jesus.

Bertelli, Timoteo. “Sopra Pietro Peregrino di Maricourt e la sua epistola de magnete. Memoria Prima.” Bullettino di Bibliografia e di Storia delle scienze matematiche e fisiche I (1868): 1–32.

———. “Sulla Epistola di Pietro Peregrino di Maricourt e sopra alcuni trovati e teorie magnetiche del secolo XIII. Memoria Seconda.” Bullettino di Bibliografia e di Storia delle scienze matematiche e fisiche I (1868): 65–139 and 319–420. Rome: Bibliotheca Scriptorum Societatis Jesu, 1676. The two contributions contain an attempt to reconstruct the contents of the not yet uncovered treatise and to assess its role in the history of magnetism.

Cabeo, Niccolò. Philosophia Magnetica in qua magnetis natura penitus explicatur. Ferrariae, 1629.

Petrus Peregrinus de Maricourt. Opera, edited by Loris Sturlese and Ron B. Thomson. Pisa: Scuola Normale Superiore, 1995. It contains the Epistula de magnete (1269), the only organic treatment of magnetism circulating before Garzoni’s time (and well known to Garzoni).

Ugaglia, Monica. “The Science of Magnetism before Gilbert: Leonardo Garzoni’s Treatise on the Loadstone.” Annals of Science 63 (2006): 59–84.

Zucchi, Niccolò. Philosophia magnetica per principia propria proposita et ad prima in suo genere promota. Rome: Biblioteca Nazionale Vittorio Emanuele II, Fondo Gesuitico 1323.

Monica Ugaglia