(b. Palermo, Sicily, 13 July 1826; d. Rome, Italy, 10 May 1910),
Cannizzaro was the youngest of the ten children of Mariano Cannizzaro, a magistrate and minister of police in Palermo, and Anna di Benedetto, who came from a family of Sicilian noblemen. Sicily was under the rule of the Bourbon kings of Naples, and the Cannizzaro family supported the regime. One of Stanislao’s sisters became a lady-in-waiting to the queen. On his mother’s side, however, there were a number of political liberals. Three of Cannizzaro’s maternal uncles were later killed in the campaigns of Garibaldi, and he himself became a strong antimonarchist.
Cannizzaro’s early education in the schools of Palermo was essentially classical, although it included some mathematics. In 1841 he entered the University of Palermo as a medical student. Here he met the physiologist Michele Foderà, who introduced him to biological research. With Foderà he attempted to work out a distinction between centrifugal and centripetal nerves. In the course of this work Cannizzaro realized his need for more understanding of chemistry, which was very poorly taught at the university.
In 1845, at the Congress of Italian Scientists in Naples, Cannizzaro reported the results of his physiological studies and met the physicist Macedonio Melloni, in whose laboratory he worked for a short time. He confided his lack of chemical training to Melloni and as a result was introduced to Raffaele Piria, professor of chemistry at the University of Pisa and the leading Italian chemist of the day. He took Cannizzaro as his laboratory assistant, not only teaching him chemistry but also allowing him to take part in investigations of natural substances. It was at Pisa, between 1845 and 1847, that Cannizzaro decided to devote himself to chemistry. Here also he became a close friend of Cesare Bertagnini, a very promising pupil of Piria’s. Although Bertagnini died at thirty, he and Cannizzaro, along with Piria, were influential in founding an Italian school of chemistry during the early 1850’s.
In the summer of 1847 Cannizzaro returned to Palermo, intending to resume his studies at Pisa in the autumn. He soon found that a revolution against the Bourbons was in preparation; and in spite of the conservatism of his family, he joined the revolutionaries. In January 1848 the Bourbons were driven from Naples and the kingdom of Sicily was established. Young Cannizzaro became an artillery officer and a representative in the Chamber of Commons and took an active part in the fighting. When the rebellion finally failed in April 1849, he was forced to flee to Marseilles.
From Marseilles he made his way to Paris, where, through the influence of Piria, he met Cahours, who introduced him into Chevreul’s laboratory in the Jardin des Plantes. Here he resumed his chemical studies, working with Stanislaus Cloëz on cyanamide and its derivatives.
In 1851 Cannizzaro was able to return to Italy as professor of physics, chemistry, and mechanics at the Collegio Nazionale in Alessandria. Although the facilities were poor, Piria urged him to accept the position because it could—and indeed did—lead to better appointments. Cannizzaro built up the research laboratory and carried out some of his best work in organic chemistry there.
As a result of his work at Alessandria, Cannizzaro was appointed professor of chemistry at the University of Genoa in 1855. There was no laboratory at the university; and Cannizzaro, an excellent teacher, was able for a time to devote much thought to his course in theoretical chemistry. It was from Genoa that in 1858 he sent the letter describing the course on which his fame chiefly rests. In September 1860 he attended the Karlsruhe Congress, at which he made known his ideas to the chemical world. In 1856 or 1857 Cannizzaro married, in Florence, Henrietta Withers, the daughter of an English pastor. They had one daughter and one son, who became an architect.
Political events again changed the course of Cannizzaro’s career. Garibaldi’s Sicilian revolt in 1860 was successful, and Cannizzaro returned to his native Palermo to take part in the new government. This time he did not participate in the actual fighting, but he became a member of the Extraordinary Council of the state of Sicily. In 1861 he was appointed professor of inorganic and organic chemistry at the University of Palermo. Once more he had to organize and build a laboratory, since the only facility for chemical research was the same small room that had been available in his student days. Cannizzaro was so successful in his efforts that Palermo became the center of chemical education in Italy. Such men as Wilhelm Körner, who devised the method of locating the position of substituents in the benzene ring, and Adolf Lieben, later a noted organic chemist in Vienna, were among his students. At the same time he was active in establishing schools of various types in Palermo, and during an epidemic of cholera he served as commissioner of public health.
With the unification of Italy, Cannizzaro made his last move, to the University of Rome in 1871. As before, he found that laboratory facilities had been neglected. He therefore founded the Italian Institute of Chemistry in the old Convent of San Lorenzo. In the functioning laboratory that he established he was able to continue the work on the constitution of natural substances that he had begun with Piria. His efforts during the latter part of his life were devoted to determining the structure of santonin, which he showed to be one of the few natural compounds derived from naphthalene. With his move to Rome, Cannizzaro was made a senator of the kingdom. As in Palermo, he spent much time on public and civic duties.
Cannizzaro continued to give his lectures with great enthusiasm and success until nearly the end of his life, discontinuing them only the year before his death at eighty-three. During the latter part of his life he was honored by most of the important scientific societies of Italy and the rest of Europe. On the centenary of his birth in 1926, during the Second National Italian Congress of Pure and Applied Chemistry, his body was transferred to the Pantheon at Palermo.
Cannizzaro carried out all of his experimental work in the field of organic chemistry. Whenever he had a laboratory available, he continued the work on natural substances that he had begun at Pisa. He also devoted much time to the study of aromatic alcohols, a class of compounds little known before his work. In 1853, while studying the behavior of benzaldehyde, he discovered its reaction with potassium hydroxide, in which an oxidation-reduction produces both benzoic acid and benzyl alcohol. This is still known to organic chemists as the “Cannizzaro reaction.” He was also the first to propose the name “hydroxyl” for the OH radical.
Cannizzaro’s lasting fame depends, however, on the letter that he wrote in 1858 to his friend Sebastiano de Luca, who had succeeded Bertagnini in Piria’s chair at Pisa. This was the famous “Sunto di un corso di filosofia chimica fatto nella Reale Università di Genova,” published in the journal Nuovo cimento, established at Pisa by Piria, in the same year and reprinted as a pamphlet in 1859. It has frequently been republished and translated.
The complicated condition of chemistry that led Cannizzaro to compose his letter stemmed from events and personalities going back as far as fifty years before the “Sunto” appeared. When Dalton published the first volume of the book explaining his atomic theory in 1808, he considered but rejected the idea that equal volumes of gases under the same conditions contained equal numbers of particles. Only a few years later, in 1811, Amedeo Avogadro took up this idea. By making a clear distinction between atoms (which he called “elementary molecules”) and molecules (“integral molecules”), he was able to draw a number of important conclusions. Three years later Ampère proposed a similar idea. If the conclusions deduced from this hypothesis had been accepted at the time they were suggested, chemists would have been spared half a century of confusion. However, the papers were not well understood; and the chemical facts known were not sufficient to provide all the evidence needed to confirm the hypothesis. Even more important, the authorities who dominated chemical thinking during the first half of the nineteenth century, Berzelius and Dumas, did not accept the idea.
Berzelius did not distinguish atoms from molecules, speaking indifferently of an atom of hydrogen or an atom of alcohol. His electrochemical (dualistic) theory, to which he tried to make all facts conform, required that chemical compounds be held together by opposite electrical charges. Thus, there could not be combination of electrically similar atoms, and hydrogen and oxygen could not be diatomic. Berzelius’ analytical determinations of atomic weights were based on Gay-Lussae’s law of combining volumes of gases and were in most cases quite accurate; however he was unable to apply this law consistently to solid compounds, and so a number of his values for atomic weights were incorrect.
Dumas recognized that vapor density determinations could be used for determining atomic weights; but since he too confused atoms and molecules, he wrote of water as composed of “an atom of hydrogen” and “half an atom of oxygen.” (To Berzelius the concept of half an atom was ridiculous.) Dumas determined the vapor densities of mercury, phosphorus, arsenic, and sulfur and found “atomic” weights that he believed were impossibly high. He therefore discarded Avogadro’s hypothesis. In 1843 Berzelius accepted Dumas’s experimental results and definitely rejected the Avogadro concept. The influence of these two men was so strong that the hypothesis of atomic weights had little chance of being accepted.
In the meantime, in 1813, Wollaston had proposed the use of equivalent weights as the fundamental units of chemistry. Equivalent weights appealed to many chemists because they seemed to be experimentally determinable without recourse to any theory. Confusion was increased because there was no standardization of meaning for many formulas employed to represent chemical compounds. Symbols involving barred or double atoms came to mean different things to different chemists. When Laurent and Gerhardt tried in the 1840’s to return to Avogadro’s principle, they went too far and introduced new confusion into chemistry. A few men, such as M. A. A. Gaudin, a calculator in the Bureau des Longitudes in France, appreciated the Avogadro hypothesis and published work depending on it; but they were outside official circles and had no influence.
Thus, when Cannizzaro wrote the “Sunto”, there was no agreement among chemists as to what values should be adopted for atomic, molecular, or equivalent weights; no possibility of systematizing the relationship of the various elements; and no unanimity as to how organic compounds should be formulated.
The deficiency in laboratory facilities in the various universities in which he had taught and his own enthusiasm for teaching had combined to cause Cannizzaro to devote much thought to the courses he gave. He well recognized the difficulty his students encountered in learning chemistry when they found that even masters of the science could not agree as to what constituted the fundamental structure of chemical compounds. Believing that he understood how this confusion had arisen, he set himself to explain as simply and clearly as he could what the true basis of chemistry should be. His being an Italian perhaps permitted Cannizzaro to see more clearly than foreign chemists what his countryman Avogadro had suggested almost fifty years earlier. In his theoretical course he now proposed to clear up the difficulties that had arisen. His letter to Luca outlined the development of his pedagogical ideas.
Cannizzaro was well-read in the history of chemistry and was therefore able to develop his course historically. He not only gave credit to the work of well-known figures but also devoted time to such little-known authors as Gaudin. His first four lectures were purely historical, to give his students the background for understanding the current situation of chemistry.
Cannizzaro began by stressing the distinction between atoms and molecules made by Avogadro and Ampère. He then explained the theories of Berzelius and how they had misled the master analyst. He also showed how Dumas had felt forced to conclude that there were different rules governing inorganic and organic chemistry. He reviewed the contributions of many chemists closer to his own time, showing how often they had approached the truth without realizing it completely. Throughout this, historical review he repeatedly insisted that application of Avogadro’s hypothesis explained the inconsistencies noted by others and that no facts contradicting it were known.
He was then ready, in his fifth lecture, to show how Avogadro’s hypothesis could be used. Most of what he pointed out had been stated, or at least implied, by Avogadro; but Cannizzaro brought it out much more clearly and was able to supply a wealth of examples from cases that had not been known earlier. He stressed that since all atomic weights are relative, one standard weight had to be chosen with which all other values could be compared. He chose hydrogen as this standard, but since he knew it to be diatomic, he used “half a molecule of hydrogen” as unity. In using this term he avoided Dumas’s error, the “half atom of hydrogen” that had so disturbed Berzelius.
Cannizzaro next told his students, “Compare the various quantities of the same element contained in the molecule of free substance and in those of all its different compounds, and you will not be able to escape the following law: The different quantities of the same element contained in different molecules are all multiples of one and the same quantity, which, always being entire, has the right to be called an atom.” This he called the law of atoms, and Partington says that it deserves to be called the Cannizzaro principle. He gave numerous examples of the application of this law, especially to metals, the atomic weights of which were in a particular state of confusion.
The method of determining molecular weights by the use of vapor densities depended on the existence of volatile compounds. When such compounds were not known for a given element, Cannizzaro used analogies or depended on the relation between atomic weight and specific heat discovered by Dulong and Petit. In the case where both methods could be used, he showed that they gave the same result. This strengthened his argument. In his discussion of organic radicals Cannizzaro stressed their similarity in combining power to atoms of various elements. This approach came very close to a statement of the theory of valence, which had not yet been clearly enunciated. He pointed out that radicals like methyl are monatomic, like hydrogen, while radicals like ethylene resemble mercuric or cupric compounds. “The analogy between mercuric salts and those of ethylene or propylene has not been noted, so far as I know, by any other chemist.”
Thus, in his “Sunto” Cannizzaro not only called attention once more to Avogadro’s hypothesis, made the distinction between atoms and molecules fully clear, and showed how vapor densities could be used to determine molecular weights (and atomic weights), but he laid to rest completely the idea that inorganic and organic chemistry functioned by different rules. As Tilden summed up his work in the Cannizzaro Memorial Lecture to the Chemical Society, “There is, in fact, but one science of chemistry and one set of atomic weights.”
When the “Sunto” was first published, it attracted little attention, possibly because of the place and language of its publication. Chemists grew more and more frustrated in their attempts to systematize their science. This was particularly true of the younger workers, who were most active in research and who most felt the need for a sound theoretical background for their studies. A leading spirit in this search for a background was August Kekulé, who had just published his epochal paper on the linking of carbon chains and the tetratomicity of carbon. In the spring of 1860 he proposed to his friend Carl Weltzien, professor of chemistry at the Technische Hochschule in Karlsruhe, that an international congress of chemists be called to establish, among other things, more precise definitions of the concepts of “atom, molecule, equivalent, atomicity, alkalinity, etc.” In association with Charles Wurtz of Paris, Kekulé and Weltzien organized the first international chemical congress, which met at Karlsruhe for three days, beginning on 3 September 1860. Most of the men attending were the younger chemists, active in research and therefore anxious to clarify the basis of their studies. Many of the well-established older men, such as Liebig and Wöhler, more sure of their theoretical ideas, did not come. Dumas was the most important of the older workers who did attend, but he spent much of his time reiterating the idea of the difference between inorganic and organic chemistry.
On the first day of the meeting, discussion centered on the distinction between physical molecules, which meant particles of a gas, liquid, or solid; chemical molecules, the smallest part of a body taking part in a reaction but capable of being divided; and atoms, which could not be divided. Although Kekulé supported this distinction, Cannizzaro stated that he could see no difference between physical and chemical molecules. On the second day questions of nomenclature were discussed, and on the third day there was a lively consideration of whether the principles of Berzelius should be adopted for purposes of nomenclature. Cannizzaro delivered a lengthy refutation of this proposal in which he summarized the arguments he had used in the “Sunto.” He strongly defended Avogadro’s hypothesis and pointed out that anomalous vapor pressures of some substances could be explained by the phenomenon of dissociation at higher temperatures, which had recently been discovered by Deville. In the discussion that followed, the prevailing opinion was that no vote could be taken on scientific questions and that each scientist should be allowed full freedom to use the system he preferred.
Cannizzaro left at the end of the meeting, probably feeling that his efforts had been futile. However, his friend Angelo Pavesi, professor of chemistry at the University of Pavia, remained behind and distributed copies of the “Sunto” which Cannizzaro had brought with him. This was the decisive step, for it brought Cannizzaro’s clear and logical arguments to the attention of the chief chemists of the day. Since these arguments had been prepared to introduce students to chemistry, they omitted no step in the reasoning or deductions and thus were ideally suited to convince even practicing chemists whose preconceptions might have prevented them from following a more condensed version.
One of the first to see the significance of the paper was Lothar Meyer, who read the pamphlet on his way back to Breslau. As he expressed it, the scales fell from his eyes and he was convinced. His book Die modernen Theorien der Chemie, published in 1864, utilized Cannizzaro’s ideas throughout and exerted a strong influence on the chemical world. Mendeleev also attended the congress and later wrote of the defense that Cannizzaro had presented for Avogadro’s hypothesis. It was the recognition of true atomic weights that permitted Meyer and Mendeleev to formulate the periodic law at the end of the 1860’s.
In organic chemistry the confusion of formulas that had originated in the disagreement over whether to use atomic or equivalent weights of carbon and oxygen also disappeared. The way was opened for the full development of the structural theory developed by Butlerov and others in the decade following the Karlsruhe Congress. In 1860 the chemical world was ready for the revival of Avogadro’s hypothesis, but it was the great logic and clarity of Cannizzaro’s presentation that made its acceptance easy.
I. Original Works. There is a bibliography of Cannizzaro’s papers on experimental chemistry in Bulletin. Société chimique de France, 4th ser., 7 (1910), VII–XIII. The Cannizzaro reaction is described by Cannizzaro himself in “Ueber den der Benzoësäure entsprechenden Alkohol,” in Justus Liebig’s Annalen der Chemie, 88 (1853), 129–130; 90 (1854), 252–254. “Sunto di un corso di filosofia chimicu fatto nella Reale Università di Genova” appeared in Nuovo cimento, 7 (1858), 321–366, and was republished as a pamphlet (Pisa, 1859). An English translation is Alembic Club Reprints, no. 18 (Edinburgh, 1910); and a German translation is Ostwald’s Klassiker der Exacten Wissenschaften, no. 30 (Leipzig, 1891).
II. Secondary Literature. Extensive biographical material is in W. A. Tilden. “Cannizzaro Memorial Lecture,” in Journal of the Chemical Society. 101 (1912). 1677–1693; and Domenico Marotta, “Stanislao Cannizzaro,” in Gazetta chimica italiana, 69 (1939), 689–717. A shorter biography is A. Gautier, “Stanislas Cannizzaro,” in Bulletin. Société chimique de France, 4th ser, 7 (1910). I–VI. Cannizzaro’ part in the Karlsruhe Congress is described by Clara de Milt, “Carl Weltzien and the Congress at Karlsruhe,” in Chymia, 1 (1948). 153–169.
Henry M. Leicester
Born in Palermo, Sicily, in 1826, Stanislao Cannizzaro began medical studies at the University of Palermo before moving to Pisa to study chemistry. However, when the Sicilian revolt broke out in 1848, Cannizzaro took part in the capture of Messina. The failure of the revolt forced Cannizzaro to flee to France, where he continued his studies in chemistry at the laboratory of Michel-Eugène Chevreul. In 1851, he returned to Italy and accepted a teaching position in Alessandro.
It was at Alessandro that Cannizzaro completed his studies of the reaction in which the action of alkali on benzaldehyde generates benzoic acid and benzyl alcohol, which came to be known as the Cannizzaro reaction. Further studies on the reaction revealed that it could employ any aldehyde without a hydrogen atom on the carbon closest to the carbonyl group.
Cannizzaro was a talented organic chemist and an early leader in the discipline, but it is his contribution to the then-existing debate over atoms, molecules, and atomic weights for which he is best known. He championed Amedeo Avogadro's notion that equal volumes of gas at the same pressure and temperature held equal numbers of molecules or atoms, and the notion that equal volumes of gas could be used to calculate atomic weights. In so doing, Cannizzaro provided a new understanding of chemistry.
During the early 1800s, many chemists were weighing in on atomic theory debates, including the debate over the actual, corporeal existence of atoms, with varying degrees of cogency. One difficulty arose from the fact that chemical formulas could be written in any one of a number of ways, if the atomic weights were unknown. For example, the formula for ordinary water, which we know is H2O, was often written "HO." Use of the latter formula required a belief that oxygen was only eight times as massive as hydrogen, and not sixteen times. The establishment of atomic weights was an important goal of chemists, as all chemical formulas were determined by ratios of elements, leading to empirical and not molecular formulas. Further, some of the leading chemists of the day had their own idiosyncratic theories on how atoms interacted, and there remained the belief that atoms did not actually exist.
In 1858 Cannizzaro published his "Sketch of a Course in Chemical Philosophy" in the journal Il Nuovo Cimento, in which he undertook to resolve many of chemistry's outstanding issues, basing his arguments on the work of Avogadro. Cannizzaro felt strongly that a consensus on these issues needed to be achieved. But the paper had little effect at the time of its publication. In 1860 August Kekulé, with the help of Adolphe Wurtz and Carl Weltzien, presided over the first international congress of chemists in Karlsruhe, Germany. Over 140 chemists, including Cannizzaro, attended the three days of discussion and debate. It was the organizers' hope that the congress would resolve some of the aforementioned issues. A participant posed the basic question: "Shall a difference be made between the expressions 'molecule' and 'atom' such that a molecule be named the smallest particle of bodies which can enter into chemical reactions … atoms being the smallest particles of those bodies which are contained in molecules?" (DeMilt, p. 38). Cannizzaro was present at the conference and spoke at length on virtually every subject that was debated.
Little was resolved at Karlsruhe. Still, in the words of attendee Dimitri Mendeleev, Jean-Baptiste-André Dumas made a brilliant speech proposing to use the new atomic weights only in organic chemistry, leaving the old for inorganic. Against this Cannizzaro spoke heatedly, showing that all should use the same new atomic weight . There was no vote on this question, but the great majority took the side of Cannizzaro.
It was after his departure from the conference that Cannizzaro finally made his point and settled the atomic weight debate. Angelo Pavesi, professor of chemistry at the University of Pavia and Cannizzaro's friend, distributed a pamphlet at the conference that contained Cannizzaro's 1858 "Sketch of a Course in Chemical Philosophy." The time was right for those in attendance to at last recognize that a systematic approach to atomic and molecular weights based on the work of Avogadro would resolve many of the outstanding disagreements. The Karlsruhe conference and Cannizzaro's paper are inevitably linked and represent a major turning point in our understanding of chemistry.
see also Atoms; Avogadro, Amedeo; Chevreul, Michel; KekulÉ, Friedrich August, Mendeleev, Dimitri; Periodic Table.
Todd W. Whitcombe
Cobb, Cathy, and Goldwhite, Harold (1995). Creations of Fire: Chemistry's Lively History from Alchemy to the Atomic Age. New York: Plenum.
DeMilt, Clara (1965). "The Congress at Karlsruhe." In Selected Readings in the History of Chemistry, ed. Aaron J. Ihde and William F. Kieffer. Easton, PA: Division of Chemical Education of the American Chemical Society.
Ihde, Aaron (1964). The Development of Modern Chemistry. New York: Harper & Row.
Partington, J. R. (1989). A Short History of Chemistry. New York: Dover Publications.