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Dulong, Pierre Louis

Dulong, Pierre Louis

(b. Rouen, France, 12/13 February 1785; d. Paris, France, 19 July 1838)

chemistry, physics.

Dulong’s father died shortly after his birth; and when he was four and a half, his mother died. An aunt, Mme. Fauraux, assumed responsibility for the young orphan and took him into her home at Auxerre, where he attended the local collège. His teachers encouraged his mathematical ability, and he was able to study some science at the école centrale founded in Auxerre in 1796. He succeeded in the competition for the École Polytechnique in Paris, which he entered in 1801 at the minimum age of sixteen. Excessive study ruined Dulong’s health, and in his second year he withdrew from the school. He then turned to medicine as a career. He could do this because formal qualifications were not required in post-Revolutionary France. Dulong practiced in one of the poorer districts of Paris. Although the number of patients rapidly increased, his small capital ran out, for the tenderhearted doctor not only treated his patients without charge but offered to pay for their prescriptions.

After leaving medicine, Dulong turned first to botany and then to chemistry, hoping to make a name for himself in this newly famous science. In chemistry he found his métier, although his subsequent work in physics and physical chemistry exhibited the value of his mathematical training at the École Polytechnique. First Dulong obtained a position as an assistant in Louis Jacques Thenard’s laboratory, and then Berthollet offered him a place in his private laboratory at Arcueil.

Dulong married Émélie Augustine Rivière on 29 October 1803. They had four children, one of whom died in infancy. Dulong’s mature years were dominated by a conflict between his desire to do research and the necessity of accepting numerous teaching, examining, and administrative positions in order to buy apparatus and provide for his family. This conflict often developed into a crisis because of his persistent bad health. His first teaching post (1811) was at the École Normale, and in 1813 he was appointed to teach chemistry and physics at the École Vétérinaire d’Alfort. In 1820 he was appointed professor of chemistry at the Faculté des Sciences in Paris. At the École Polytechnique he was first appointed as examiner (1813–1820) and then professor of physics (1820–1830). Of the latter appointment Dulong wrote:

Through a weakness of character for which I reproach myself incessantly, I have consented to accept the professorship of physics at the École Polytechnique, which the death of my unfortunate friend [Petit] has left vacant. Even with good health the duties of this post would have left me with little free time, so judge how much of this I have had, sick as I have been for the past eighteen months.1

In 1831 Dulong’s friend Arago succeeded in relieving him of his teaching duties at the École Polytechnique by securing for him the post of director of studies, an office that was purely administrative. Dulong’s health subsequently improved, but he bitterly regretted that he no longer had the laboratory facilities of his teaching post.

Dulong was a member of the Société d’Arcueil (1811) and the Société Philomatique (1812). He was elected to the physics section of the Académie des Sciences on 27 January 1823. In 1828 he became president of the Academy for one year, and later ill-advisedly accepted the position of permanent secretary for a short time (1832–1833).

Dulong’s first publication, a report on work that he had carried out at Arcueil, reflects very clearly the influence of his patron, Berthollet. This memoir, read at a meeting of the First Class of the Institute in July 1811, provided a striking confirmation of Berthollet’s thesis that chemical affinities were not fixed and that chemical reactions could often be reversed. Dulong showed that when barium sulfate, a notoriously “insoluble” salt, was boiled with a solution containing an equivalent quantity of potassium carbonate, it was partly decomposed. In another experiment, in which ”insoluble” barium carbonate was added to a boiling solution of potassium sulfate, a partial exchange took place. From the small amount of barium sulfate formed, Dulong considered that a state of equilibrium had been reached.

It was his work on “insoluble” salts that lẹd Dulong to make a special study of the oxalates of barium, strontium, and calcium. He went on to study the action of heat on oxalates and reached the conclusion that oxalic acid consisted of hydrogen and carbon dioxide;2 he suggested that it might accordingly be renamed “hydrocarbonic acid.” To Dulong, a metal oxalate was a simple compound of the metal and carbonic acid, although this was contrary to current chemical theory, according to which metals could combine with acids only in the form of oxides. Dulong’s work therefore contributed to the post-Lavoisier conception of an acid as a compound containing hydrogen that is replaceable by a metal. Dulong compared the carbon dioxide in a metal oxalate to cyanogen in a cyanide or chlorine in hydrochloric acid. He thus deserves mention as a precursor of the radical theory in organic chemistry.

Better known, and certainly more spectacular, was Dulong’s discovery of the spontaneously explosive oil nitrogen trichloride. Not realizing the danger involved when he first prepared a small quantity of the substance in October 1811, he lost a finger and the sight of one eye. He began his memoir (1813) by pointing out that only three elements—nitrogen, carbon, and boron—were apparently unable to combine with “oxymuriatic acid” (chlorine). He presented his research as an attempt to make nitrogen and chlorine combine. Although the gases did not combine directly, Dulong was more successful when he passed a current of chlorine through a fairly concentrated solution of ammonium chloride. After the reaction had been going on for two hours at a temperature of between 7° and 8° C., a yellow oil began to form. This was what exploded

By February 1812 Dulong had sufficiently recovered from his injuries to resume his investigation. He now realized that he could avoid an explosion only if he kept the temperature low. Accordingly, he waited until the following October to carry out further research. He succeeded in determining the qualitative composition of the oil by allowing it to come into contact with copper; the only products were copper chloride and nitrogen. After receiving further injuries, Dulong abandoned this research. His memoir, however, contains the following remarks about the explosive properties of the new compound. They are significant in the light of his later thermochemical studies:

It seems to me that this compound contains a certain amount of combined heat which, when its elements separate from each other, raises their temperature and imparts to them a great elastic force.3

In 1816 Dulong carried out two pieces of chemical research prompted by the interest of Berthollet’s circle at Arcueil in combining proportions. There was very great divergence between the analyses of phosphoric acid published by various chemists, and there was also some difference of opinion between Gay-Lussac and Thenard on the composition of phosphorous acid. It was in an attempt to clarify this situation that Dulong found that there were at least four acids of phosphorus. In addition to the two mentioned above, the composition of which he determined accurately, Dulong was able to confirm the existence of hypophosphoric acid and discovered a fourth acid that was obtained from the solution remaining after the action of water on barium phosphide. Dulong named this syrupy acid “hypophosphorous acid.” His memoir (1817) contained a careful analysis of the new acid and a discussion of its salts.

The various oxides of nitrogen had created some confusion in the early nineteenth century. In 1816 Gay-Lussac had succeeded in distinguishing five oxides of nitrogen and had given their correct chemical composition. Dulong repeated Gay-Lussac’s preparation of dinitrogen tetroxide. By heating dry crystals of lead nitrate, he excluded water from the product, which was collected as a liquid in a tube surrounded by a freezing mixture. He was the first to make a study of the color changes undergone by this interesting compound over a wide range of temperature, from a colorless solid at -20°C. to a deep red vapor when heated.

In 1819, when Berzelius was in Paris, Dulong collaborated with him in determining the gravimetric composition of water. This was a fundamental datum of chemistry, and it was important that it should be determined with great precision. They passed pure hydrogen over heated copper oxide and absorbed the water formed with anhydrous calcium chloride. The mean of their results gave the ratio H: O = 11.1:88.9, or, as Dumas later represented it, 1:8.008. Dulong and Berzelius had been able to work only to an accuracy of approximately 1/60, and the remarkable accuracy of their result was due largely to the canceling out of errors. Dumas did not carry out his classic redetermination of this ratio until 1842.

In 1815 Dulong’s famous collaboration with the mathematical physicist Alexis Thérèse Petit began; it produced three important memoirs on heat. The best-known part of this work is the statement of the law of constant atomic heats that bears their names, which is discussed further below. They began with the fundamental problem of measuring quantities of heat, which involved a critical analysis of thermometric scales. In 1804–1805 Gay-Lussac had carried out a comparison of mercury and air thermometers between 0°C. and 100°C. Dulong and Petit extended the range of comparison up to 300°C. and found an increasing discrepancy between the two scales at higher temperatures.

Dulong and Petit continued their researches in 1817, stimulated by the subjects of the prize to be awarded by the Académie des Sciences in 1818. The first of the three subjects for this prize was to determine the movement of the mercury thermometer as compared with an air thermometer from –20°C. to + 200°C. They approached the subject by determining the absolute coefficient of expansion of mercury, and to do this they introduced the now classic method of balancing columns. Two vertical columns of mercury, one hot and the other cold, were connected by a thin horizontal tube. Since the columns balanced, the two pressures were equal, or

h:d = h’:d’

where h is the height of the column and d is the density. Since density is inversely proportional to volume, a simple method was now available for direct measurement of the expansion of the mercury without reference to the material of the vessel. A refined version of this apparatus, introduced later by Regnault, became the standard apparatus for determining a liquid’s coefficient of absolute expansion.

The second part of the subject for the Academy’s prize was to determine the laws of cooling in a vacuum; Dulong and Petit accordingly undertook a complete re-examination of Newton’s law of cooling. The most remarkable feature of their work was the way they broke down a complex phenomenon into its constituent parts and dealt with each factor separately. For example, they distinguished losses due to radiation from those due to contact with particles of a gas. They thus arrived at a series of laws relating to different special cases. It was not until 1879 that Stefan was able to reduce the phenomenon of radiation to a simple law. The memoir in which he did this took the work of Dulong and Petit as its starting point, and Stefan was at pains to show that his law agreed with their experimental results.4 There is no doubt that the young Frenchmen deserved to win the 3,000franc prize offered by the Academy.

The third and last joint memoir of Dulong and Petit was also on heat. Among its far-reaching implications was a new approach to Dalton’s atomic theory, which had been received in France with deep skepticism. In considering some of the implications of the atomic theory, Dulong and Petit therefore began on a defensive note:

Convinced.... that certain properties of matter would present themselves under more simple forms, and could be expressed by more regular and less complicated laws, if we could refer them to the elements upon which they immediately depend, we have endeavoured to introduce the most certain results of the atomic theory into the study of some of the properties which appear most intimately connected with the individual action of the material molecules.5

They were concerned with the specific heats of elements; but if these elements really existed as atoms, it seemed possible that there might be a connection between the weight of the atom and the amount of heat required to raise the temperature of a given weight of that element by a certain amount.

Dulong and Petit first had to develop a reliable method of determining specific heats, for the published data were quite unreliable. They adopted the method of cooling that used the finely powdered solid packed round the bulb of a thermometer (since the rates of cooling are directly proportional to the thermal capacities and hence to the specific heats). They recorded the specific heats of a dozen metals and sulfur, and then multiplied each by the element’s atomic weight. The following table shows the remarkably constant value obtained.

 Specific heat (water = 1)Atomic weight (oxygen = 1)product of specific heat and atomic weight

Dulong and Petit said that inspection of this table showed “the existence of a physical law susceptible of being generalized and extended to all elementary substances.” Certainly it showed that the specific heats of the elements tested were inversely proportional to their atomic weights. Their interest in the atomic structure of matter is revealed by their conclusion, “The atoms of all simple bodies have exactly the same capacity for heat,” and also by their suggestion that the actual distances between the atoms might be calculated from thermal expansion data. Of more immediate concern to chemists, however, was the use of their law of atomic heats in settling disputed values of atomic weights. On the basis of their law, Dulong and Petit changed some of Berzelius’ atomic weights—for example, they halved his values for silver and sulfur. Following his success in relating atomic weights to specific heats, Dulong investigated the possibility of a relation between the atomic (or molecular) weights of gases and their refractive indexes, but with little success.

After Petit’s death in 1820, Dulong carried out further research on heat by himself and published a memoir on the specific heats of gases (1829). He determined the relation of the specific heats of various gases at constant pressure and constant volume by measuring the effect of change in temperature on the tone produced when the respective gases were passed through a flute. His method was an extension of one used by Chladni in 1807. He concluded that (1) equal volumes of all gases under the same conditions of temperature and pressure, when suddenly compressed or expanded to the same fraction of their original volume, give off or absorb the same quantity of heat; (2) the resulting temperature changes are inversely proportional to the specific heats of the respective gases at constant volume.

Dulong also worked on animal heat, taking up the subject when it was chosen in 1821 for the prize of the Académie des Sciences. He devised a respiration calorimeter in which the heat was absorbed by water rather than by ice, as in the classic apparatus of Lavoisier and Laplace. Because of unsatisfactory agreement between the theoretical and actual quantities of heat obtained (due largely to incorrect data), Dulong was not satisfied with his work and it was not published until after his death. The situation was similar in the case of his work on thermochemistry, where he measured several heats of combustion. In conversations with Hess in 1837, Dulong made such generalizations as “The quantities of heat evolved are approximately the same for the same substances, combining at different temperatures”, and “Equal volumes of all gases give out the same quantity of heat.” Dulong’s concern with generalizing about heats of reaction may have inspired Hess to formulate his law of constant heat summation (1840).

Dulong collaborated with Thenard on a study of catalytic phenomena. They confirmed Döbereiner’s discovery that a jet of hydrogen could be kindled by allowing it to impinge in air on spongy platinum. They found that palladium, rhodium, and iridium are active at room temperature and other metals at higher temperatures. They realized that the metal’s activity was dependent on its physical state but offered no explanation of the action of these substances, which Berzelius later called catalysts.

Dulong collaborated with Arago on a long and perilous study of the pressure of steam at high temperatures. This work was prompted by the French government’s concern about the safety of boilers. They first confirmed the validity of Boyle’s law at pressures up to twenty-seven atmospheres. They were then able to measure the steam pressure in boilers by means of a manometer, and afterward they calculated the temperatures corresponding to these pressures.


1. Letter to Berzelius, 21 Aug. 1821, in Berzelius, Bref (Uppsala, 1912–1925), II, pt. 4 , 29.

2. Although this was never published by Dulong, Cuvier gave an account of it in Mémoires de l’Institut (1813–1815), Histoire, 198–200. See also Gay-Lussac, in Annales de chimie el de physique, 1 (1816), 157; and Ampère, ibid., p. 298.

3.Mémoires de physique et de chimie de la Sociétée d’Arcueil, 3 (1817), 62.

4. “Über die Bezeihung zwischen der Wärmestrahlung und der Temperatur. I. Üiber die Versuche von Dulong and Petit,” in Sitzungsberichte der kaiserlichen Akademie der Wissenschaften. Mathematisch-naturwissenschaftliche Classe, 59 , Abt. 2 (1879), 391–410.

5.Annales de chimie et de physique, 10 (1819), 395.


I. Original Works. Dulong wrote both alone and in collaboration. Works of which he was the sole author include “Recherches sur la décomposition mutuelle des sels solubles et insolubles,” in Annales de chimie et de physique, 82 (1812), 273–308; “Mémoire sur une nouvelle substance détonnante,” ibid., 86 (1813), 37–43, and in Mémoires de physique et de chimie de la Sociéeté d’Arcueil, 3 (1817), 48–63; “Observations sur quelques combinaisons de l’azote avec l’oxigène,” in Annales de chimie et de physique, 2 (1816), 317–328; “Mémoire sur les combinaisons du phosphore avec l’oxigène,” in Mémoires de physique et de chimie de la Société d’sArcueil, 3 (1817), 405–452; “Recherches sur les pouvoirs réfringents des fluides élastiques,” in Annales de chimie et de physique, 31 (1826), 154–181; “Recherches sur la chaleur specifique des fluides élastiques,” ibid., 41 (1829), 113–158; “Sur la chaleur dégagée pendant la combustion de diverses substances simples ou composées,” in Comptes rendus de l’Académie des sciences, 7 (1838), 871–877; and “De la chaleur animale” (1822–1823), in Annales de chimie et de physique, 3rd ser., 1 (1841), 440–455.

With Petit, he wrote “Lois de la dilatation des solides, des liquides et des fluides élastiques à de hautes températures” in Annales de chimie et de physique, 2 (1816), 240–264; “Recherches sur la mesure des températures, et sur les lois de la communication de la chaleur,” ibid., 7 (1817), 113–154, 225–264, 337–367; and “Recherches sur quelques points importants de la théorie de la chaleur.” ibid., 10 (1819), 395–413.

Berzelius collaborated with him on “Nouvelles déterminations des proportions de l’eau et de la densité de quelques fluides élastiques,” in Annales de chimie et de physique, 15 (1820), 386–395.

Dulong and Thenard’s researches produced “Note sur la propriété que possèdent quelques métaux de faciliter la combinaison des fluides élastiques,” in Annales de chimie et de physique, 23 (1823), 440–444; and “Nouvelles observations sur la propriété dont jouissent certains corps de favoriser la combinaison des fluides élastiques,” ibid., 24 (1823), 380–387.

Arago and Dulong wrote “Exposé des recherches faites par ordre de l’Académie Royale des Sciences, pour déterminer les forces élastiques de la vapeur d’eau à de hautes températures,” in Annales de chimie et de physique, 43 (1830), 74–110.

II. Secondary Literature. Works on Dulong or his contributions are F. Arago, “Dulong,” in Notices biographiques, 2nd ed. (Paris, 1865), III, 581–584; M. P. Crosland, The Society of Arcueil. A View of French Science at the Time of Napoleon I (Cambridge, Mass., 1967); R. Fox, “The Background to the Discovery of Dulong and Petit’s Law,” in British Journal for the History of Science, 4 (1968–1969), 1–22; J. Girardin and C. Laurens, Dulong de Rouen. Sa vie et ses ouvrages (Rouen, 1854); J. Jamin, “Études sur la chaleur statique et la vapeur, travaux de Dulong et Petit,” in Revue des deux mondes, 11 (1855), 377–397; P. Lemay and R. E. Oesper, “Pierre Louis Dulong, His Life and Work,” in Chymia, 1 (1948), 171–190; and G. Lemoine, “Dulong,” in Livre centenaire de l’École polytechnique (Paris, 1895), I, 269–278.

M. P. Crosland

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