Giovanni Fabbroni (Florence, 1752–Pisa, 1823) was a prominent figure in the late eighteenth- and early nineteenth-century Florentine science and politics. His father was from Florence and his mother was from Heidelberg. They were both involved in theatre. Financial difficulties prevented the family from sending Giovanni to the University of Pisa. Instead, he studied at the Accademia del Disegno and the Arcispedale di Santa Maria Nuova school in Florence, excelling in the study of natural sciences. From a young age, he was sent several times to Venice by his mentors to purchase optical instruments and laboratory glassware. In 1773, physiologist, naturalist and chemist Felice Fontana (1730–1805) was commissioned by Leopold II (1747–1792), of Habsburg-Lorraine house, Grand Duke of Tuscany from 1765 to 1790, to organise the new Regio Museo di Fisica e Storia Naturale (Royal Museum of Physics and Natural History) in Florence. The museum’s purpose was “to enlighten the people and make them happy by making them more cultured”, in accordance with the Enlightenment ideal that scientific and technical culture was fundamental to the material, social and moral progress of the entire community. Fontana asked for and obtained the help of Fabbroni, who thus entered the Crown’s permanent service. In the winter of 1775, Fontana and Fabbroni set off on a tour of Europe to purchase scientific equipment for the museum. Fabbroni spent a long time in Paris, frequenting Enlightenment circles and establishing lasting contacts with leading scientists of the time. While in Paris, he joined the grand Masonic lodge Les Neuf Soeurs, presided over in those years by Benjamin Franklin. As a fellow Mason, on 7 April 1778, he met and paid homage to Voltaire in Paris. In the summer of 1778, he moved to London and remained there until the autumn of 1779. While travelling through the northern counties of England, he visited factories producing textile machinery, porcelain and sulphuric acid, and learned modern agricultural techniques, gaining experience that would prove useful later for advancing economic activities in Tuscany. Returning permanently to Florence, in 1780 he was appointed deputy director of the Museum by the Grand Duke.

Once he had fulfilled his management and administrative duties at the Museum, which Director Fontana did not enjoy, he was able to pursue his scientific interests, ranging from agronomy to economics and chemistry. During his time in Paris, in the years of Lavoisier’s chemical revolution, Fabbroni had collaborated with Jean d’Arcet (1724–1801), professor of chemistry at the Collège de France and a member of the Académie des Sciences, as well as with Hilaire-Marin Rouelle (1718–1779), ‘démonstrateur en chimie’ at the Jardin des Plantes in Paris, the ‘discoverer’ of urea [23].

From 1777 to 1778, Fabbroni had translated Torbern Bergman’s ‘Opuscula physica et chemica’ from Latin into Italian [24]. In spring 1779, he wrote his first experimental scientific work, ‘On the Nature of Arsenic and the Manner of Preparing Arsenical Acid’, in Italian, and published it in the Italian magazine Opuscoli scelti sulle scienze e sulle arti (Selected Papers on Science and the Arts) [25].

In 1792, Fabbroni read Galvani’s Commentarius with great interest and admiration. However, he disputed the idea that electric fluid was responsible for muscle contraction, likening the human body to a Leyden jar. Instead, he proposed that the electric fluid observed in Galvani’s experiments had a chemical origin. This conclusion was based on his observation that different metals, when placed in contact in the presence of moisture, would undergo chemical reactions. Fabbroni discussed this topic in detail in a lecture entitled “The chemical action of the metals felt once again” that he delivered to the Accademia dei Georgofili on 21 August 1793.Footnote 5 Founded in 1753, the Real Società Economica di Firenze, commonly known as the Accademia dei Georgofili, was a Florentine scientific institution that promoted and coordinated studies in agronomy (‘georgophile’, from ancient Greek γεωργός, farmer, and φίλος, friend: ‘friend of agriculture’). Its aim was to foster the progress of agriculture, which was by far the most important economic activity in Tuscany at the time. As a typical prince of the Age of Enlightenment, Leopold II strongly encouraged and supported this initiative. The Academy also included physicists, chemists and economists among its members and was the only scientific academy in Florence. Fabbroni was one such member, and, in addition to presenting contributions on agronomy, he also gave lectures on topics from other disciplines in which he was interested, such as chemistry, applied chemistry, physics and economics.

In his lecture on the behaviour of metals [26], Fabbroni began by presenting evidence from everyday life that was familiar to craftsmen, but which had never been examined or interpreted by philosophers. Here are some examples: (i) pure tin retains its natural lustre, but tin solder on copper quickly calcines when exposed to air; (ii) iron nails do not degrade, but corrode rapidly when used to fix copper plates to the hull of a ship; and (iii) lead pipes used to drain liquids from kitchens to sewers corrode where they come into contact with the iron rings that fix them to the walls. Moreover, he described some simple experiments he had carried out.

In an experiment, two small plates of different metals—one tin and one silver—separated from each other, are immersed in water: the two small plates remain intact indefinitely.Footnote 6 However, if the plates are brought into contact with each other, as shown in Fig. 4a, a white solid forms on the tin plate and gradually sinks to the bottom. If the experiment is carried out in a solution with an oil layer on the surface, however, the formation of the white solid is minimal. Fabbroni correctly suggested that tin undergoes a calcination process, in which atmospheric oxygen plays an essential role. He provided a complicated explanation of the phenomenon, which we will examine later. Nowadays, we can explain the process using the language of modern chemistry, in terms of the oxidation and reduction half-reactions reported below:

$$},\;}\;\left( + \right)\quad \; \times \;(} \to }^ + }} + }^ )$$

(2)

$$},\;}\;()\quad }_} + }_} } + }^ \to }^$$

(3)

$$}\quad } + }_} + }_} } \to }\left( }} \right)_}$$

(4)

Fig. 4

a Fabbroni’s experiment: if two small plates of different metals (tin and silver) are placed in contact in an aqueous solution, tin oxidises to form a white precipitate. b The same system is arranged as a galvanic cell with an electron transfer process taking place from the zinc electrode to the inert silver electrode. For the half-reactions at the metal electrodes, see Eqs. (2) and (3). c A pile constituted of four couples of Sn/Ag discs separated by wet cardboard discs: the circuit is closed by a metal wire

Metal tin oxidises and passes into solution as Sn2+, leaving two electrons on the electrode—Eq. (2). These electrons then travel via the electric wire to the silver electrode (Fig. 4b), where they are available to an oxidising agent. The strongest oxidising agent present in the solution is the dissolved oxygen, which is reduced to OH− according to Eq. (3). The OH− ions migrate to the anode, where they combine with Sn2+ to form the white, insoluble hydroxide Sn(OH)2—overall reaction (4)—which deposits on the tin surface, then precipitates. When a layer of oil covers the surface, the dissolved oxygen in the solution reacts to form a thin white film on tin (the solubility of O2 at 25 °C and 1 atm. is 2.59 × 10−4 mol dm−3). Then, as the oil slows the supply of atmospheric oxygen, the precipitation of the hydroxide is not observed within the same observation interval as in the absence of oil (time not specified by the author). Equilibrium (4) is strongly shifted to the right, being characterised by ΔG° = − 687 kJ mol−1.Footnote 7 The illustration in Fig. 4c shows how Fabbroni’s observations could be converted in an elementary Sn–air (oxygen) battery.

It should be noted that the nature of the metal acting as the cathode is irrelevant to the thermodynamics of the process; it merely furnishes an inert support for the transfer of electrons. Thus, silver can be replaced with another metal, provided it has a standard reduction potential distinctly more positive than that of the metal acting as an anode and a reducing agent (e.g. gold). The nature of the anode, on the other hand, is decisive: the more negative the value of E°, the greater the value of ΔE°. In the final and most known version of his pile, Volta used copper as the inert cathode (instead of the more expensive silver) and zinc. The choice of zinc was suggested to Volta by its position in the electrochemical series (1) devised by himself [18, 20]. This choice was thermodynamically correct as the overall reaction 2 Zn2+ + O2 + 2 H2O → 2 Zn(OH)2 is characterised by a distinctly more negative value of ΔG° = − 794 kcal mol−1.Footnote 8

Fabbroni provided an explanation of the phenomenon in line with the chemical knowledge of the time: when two different metals are in contact, they establish a mutual attraction. This interaction weakens the cohesive forces of each metal (the same interaction, in this case much weaker, that favours their mixing upon melting to give an alloy). The two metals, activated by the reciprocal attraction, interact with water. Fabbroni, who during his stay in Paris had become familiar with the principles of the new chemistry, knew that water is composed of two elements: hydrogen and oxygen. He suggested that the more oxidisable metal (e.g. tin) attracts oxygen and the less oxidisable metal (e.g. silver) attracts hydrogen. This is followed by the decomposition of water into its elements. The oxygen reacts with tin to form white oxide, which is a clearly perceptible phenomenon. The fate of the hydrogen is unclear: it may be absorbed by the silver, or it may form a hypothetical salt of ‘hydrogenated tin’. Regardless of its nature, this compound reacts with the atmospheric oxygen dissolved in the solution to restore the decomposed water. In this way, Fabbroni explained why the phenomenon (i) occurred in water (or in any case required the presence of moisture), and (ii) it required the presence of atmospheric oxygen.

In the conclusions, Fabbroni reiterated that the muscle contractions of the frog, induced by contact with two different metals, do not indicate the presence of ‘electric fire’ in animal fibres; rather, they are a consequence of the energy released in the aforementioned chemical processes. Finally, he pointed out that also the famous experiment by Johann Georg Sulzer (1720–1779) could be attributed to a calcination (oxidation) reaction. In that experiment [27], the Swiss philosopher, having placed lead and silver plates in contact with his tongue, felt a particular sensation and tasted iron vitriol (FeSO4·7H2O). Fabbroni himself placed a tin/silver pair on his tongue and detected a taste of metal lime.

Confirmation and development of Fabbroni’s chemical hypothesis

The lectures delivered at Accademia dei Georgofili meetings were usually compiled into a volume of proceedings. Volumes were not printed on a fixed schedule, but rather when a sufficient number of papers had been collected. Fabbroni’s paper on the action of different metals did not appear in the first available volume, Vol. II, 1795, or Vol. III, 1796. Instead, it appeared in Vol. IV, 1801 [26], though it is unclear whether this was due to negligence on the part of the author or publisher.

In the final years of the century, Fabbroni, strongly encouraged by the Habsburg-Lorraine Government, became absorbed in studies and research in agronomy and economics, finalised to a modern reformation of the Tuscan agriculture. Moreover, in 1793, Archduke Ferdinand III (1769–1824), successor of Leopold II, commissioned Fabbroni to compile the Civil Code, thereby making him an active part of the Grand Duchy’s political life. Fabbroni was therefore forced to abandon experimental research, but he continued to follow scientific publications.

In 1799, witnessing the growing interest in galvanic electricity, he sent an article entitled ‘Sur l’action chimique des différens métaux entr’eux, à la température commune de l’atmosphère, et sur l’explication de quelques phénomènes galvaniques’ to the Journal de Physique, de Chimie, d’Histoire Naturelle et des Arts, edited by the French geologist Jean-Claude Delamétherie (1743–1817) [6]. The article was published in the issue of ‘Messidor an VII de la République’ (19 June–18 July 1799 in the Gregorian calendar). The article was signed only with the surname ‘Fabroni’ (with the implicit ‘citoyen’, as was customary—if not obligatory—for those living in France or states under French control). The author had deliberately used the family name with a single ‘b’ as he usually did when writing in French (a habit he had probably acquired during his long stay in Paris).

At the beginning of the paper, Fabbroni mentioned his dissertation at the Accademia dei Georgofili [26], stating that it was still awaiting publication in the proceedings volume and that he no longer had the original text. So, what was written in the article, he said, was what he remembered. In fact, the article reported on all the experiments and observations described in the lecture, and presented no new information, but it was written more clearly and in a more organised manner, and in excellent French.

The following month, a two-page résumé of the article published in the Journal de Physique appeared in the Bulletin des Sciences par la Société Philomatique dated Thermidor, an 7 de la République (19 July–18 August 1799) [28]. The approximately 1000-word résumé, entitled ‘Sur l’action chimique des différens métaux entr’eux, à la température commune de l’atmosphère’ was written by an assistant editor with the initials A. B. Apparently, the abstract arrived in London before the full paper. Nicholson translated the abstract into English and published it in the October 1799 issue of the Journal of Natural Philosophy (‘On the Chemical Action of different Metals upon each other at the common Temperature of the Atmosphere’ by Cit. Fabroni’—one ‘b’, French style) [29]. Tilloch included the abstract translated by Nicholson verbatim in the December 1799 issue of the Philosophical Magazine [30]. A few months later, the full paper also reached London. Nicholson translated this into English and published it in the June 1800 issue of his journal, with the complete title (‘On the Chemical Action of different Metals upon each other at the common Temperature of the Atmosphere, and upon the Explanation of certain Galvanic Phenomena’ by Cit. Fabbroni—two ‘b’, Italian style) [31].

In the issue of the Journal of Natural Philosophy of the following month, Nicholson published his seminal article on Volta’s pile and its use in the electrical decomposition of water into its elements [2]. In his description of the pile’s construction and operation, Nicholson expressed surprise that Volta had not considered the chemical nature of galvanism, “on which Fabbroni had so insisted.” Nicholson observed that a layer of white oxide formed on the edge of the zinc plate that was in contact with the wet cardboard. The formation of this insulating layer meant that the pile lasted only 2 or 3 days at most, which was in contrast to Volta’s idea of the perpetual circular motion of electricity [1].

Subsequent issues of the Journal of Natural Philosophy published articles on the pile that took up Fabbroni’s chemical hypothesis and confirmed it experimentally. Henry Haldane (1756–1825), a British engineer and retired lieutenant-colonel of the army, and amateur chemist living in Croydon, South London, built a pile following the instructions in the article published in the Morning Chronicle on 30 May 1800 and studied how it worked [32]. He observed that the pile ceased to function when placed in a vacuum or kept in a nitrogen atmosphere, in agreement with Fabbroni’s observation that the oxidation of tin in contact with silver takes place only in the presence of oxygen [6]. Haldane concluded his article by stating: “I think we may agree with Cit. Fabbroni that the effects of galvanism depend on a chemical operation and are principally produced by the attraction of oxygen from the atmosphere. Therefore, on the present theory, the whole operation can be considered only as combustion.” In an article studying the causes of galvanic phenomena and the ways to increase the power of Volta’s pile [34], the 22-year-old Humphry Davy, at the time Superintendent at the Pneumatic Institution in Bristol, repeated the experiments of Haldane, then observed that the pile did not work when placed in a hydrogen atmosphere or when the cardboard discs were soaked in deoxygenated water.Footnote 9 He also found that the pile became more effective (judging by the intensity of the shock) when the discs were soaked in muriatic acid. Davy finally stated: “It seems reasonable to conclude, though with our present quantity of facts we are unable to explain the exact mode of operation, that the oxidation of the zinc in the pile and the chemical changes connected with it are some how the cause of the electrical effects it produces.” And he acknowledged that “Fabroni was the first who systematically attempted to prove that they were chemical effects.”

At the Royal Society meeting on 25 June 1801, William Hyde Wollaston (FRS 1766–1828), a distinguished chemist, presented a paper entitled “Experiments on the Chemical Production and Agency of Electricity” [35]. Wollaston repeated Fabbroni’s experiments with bars of different metals immersed in solution but varied the nature of the solutions. In particular:

1.

A zinc plate was immersed in a solution of diluted muriatic acid: the zinc dissolved and hydrogen was produced, a well-known method for preparing the ‘flammable air’, recognised as a discrete substance by Cavendish in 1776 [36]. If a silver plate is immersed in the same solution, nothing changed. But if the zinc and silver plates are brought into contact, hydrogen was released on silver.Footnote 10 At the time, the dissociation of acids and the existence of H+ ions in solution were unknown. So, Wollaston imagined that the dissolution of zinc generated electricity, that this electricity was transmitted to the silver and that, through the silver, which behaved as a mere conductor (an ‘inert electrode’ ante litteram!), decomposed water. This interpretation is not too far from reality, if we make explicit the correspondence between electricity and electrons. In today’s chemical language, we would say that the Ag/Zn pair in acidic solution gives rise to a galvanic cell in which the half-reactions (5) and (6) take place, while reaction (7) illustrates the overall redox process:

$$},\;}\;\left( \right) \quad } \to }^ + }} + }^$$

(5)

$$},\;}\;( + )\quad }^ + }^ \to }_}$$

(6)

$$}\quad } + }^ \to }^ + }} + }_}$$

(7)

O2 (at pH = 0, E°(O2/H2O) = 1.23 V) is a stronger oxidant than H+ (at pH = 0, E°(H+/H2) = 0.00 V) and should therefore prevail in the uptake of electrons at the anode. However, the occurrence of the reduction process is controlled by a variety of factors: (i) concentration; (ii) overpotentials related to the interaction of O2 and H2 with electrode surface; (iii) the diffusion rate of H+ and O2 in water, whose ratio is DH+/DO2 ≈ 104–105. The balance of these varying contributions favours the reduction of H+.

2.

A copper plate was immersed in a nitric acid solution: nitrogen oxide developed on the surface of the copper plate. If, in the same solution, a gold rod is immersed in the same solution, nothing changes. However, if it is placed in contact with the copper plate, nitrogen oxide develops on both metals. The half-reactions at the electrodes and the overall reaction are shown below in today’s chemical terminology.

$$},\;}\;\left( + \right) \quad 3 \times (} \to }^ + }} + }^)$$

(8)

$$},\;}\;()\quad \quad }_}^ + }^ + }^ \to } + }_} }$$

(9)

$$}\quad \quad} + }_}^ + }^ \to }^ + }} + } + }_} }$$

(10)

3.

When an iron plate is immersed in a copper sulphate solution, metallic copper is deposited on the iron. If a silver plate is immersed in the same solution, nothing changes. However, if the two plates are placed in contact with each other, copper is deposited on the silver plate.Footnote 11 The redox processes are described below.

$$},\;}\;\left( + \right)\quad } \to }^ + }} + }^$$

(11)

$$},\;}\;()\quad }^ + }} + }^ \to }$$

(12)

$$}\quad } + }^ + }} \to }^ + }} + }$$

(13)

Through these simple experiments, Wollaston demonstrated that the electricity in Volta’s pile is not generated by the formation of metal oxides from non-noble metals (such as zinc and tin), as hypothesised by Fabbroni, Nicholson, Haldane and Davy, but rather by “the chemical action of the fluid on the metals”, an interpretation closer to the modern one.

The system of different metal plates in contact in solution (Fabbroni and Wollaston) and that of the zinc and that of silver discs separated by wet cardboard (Volta) can be traced back to a galvanic cell of the type shown in Fig. 4b. However, this is an ‘imperfect’ galvanic cell, as the two electrodes are immersed in the same solution, causing the two half-reactions to take place in the same solution and potentially interfere.

The first galvanic cell constituted by separate half-cells, that were nevertheless in electrical contact, was introduced in 1836 by John Frederic Daniell (1790–1845), a professor at the East India Company’s Military Seminary in Addiscombe, Surrey [37].

The Daniell cell was based on the overall redox reaction Cu2+ + Zn → Cu + Zn2+. The two half-cells were separated by an ox gullet that acted as a membrane, allowing ions to pass through. In a later version, the membrane was replaced by a porous earthenware tube (Fig. 5a). Figure 5b shows a cross-section of the cell. The anode, a zinc tube, is immersed in a glass vessel containing a solution of diluted sulphuric acid, which was later replaced by a zinc sulphate solution. A copper tube (the cathode; platinum would work better but is too expensive for a practical use) was lowered into the earthenware pot. The pot was filled with a sulfuric acid solution saturated in CuSO4, with an abundant amount of the salt in crystalline form. Batteries made of Daniell cells guaranteed a constant supply of electricity and were used for decades to power the telegraph network. A Daniell cell was also used to calibrate instruments such as ammeters [39]. Today, the Daniell cell is best known in its educational version (Fig. 5c), in which the two half-cells are connected by a salt bridge. This version is typically featured in any textbook of general chemistry, at the beginning of the chapter on electrochemistry.

Fig. 5

a An early version of Daniell’s cell [38]. b A cross-section of the cell. c An educational version of Daniell’s cell, in which the electric contact between half-cells is ensured by a salt bridge (a reversed glass U-tube filled with a saturated solution of KNO3)