Deluc, Jean André

(b. Geneva, Switzerland, 8 February 1727; d. Windsor, England, 7 November 1817)

geology, meteorology, physics, natural philosophy, theology.

Deluc was descended from a family who had emigrated from Lucca, Tuscany, and had settled in Geneva, probably in the fifteenth century. His father, François, refuted the satirical ideas of Bernard Mandeville and others in several treatises that were known to Rousseau, who wrote a diverting description of how much they had bored him.

Deluc received an excellent education, particularly in mathematics and natural science. He then took up commerce, which he combined with political activities. In 1768 he went to Paris on a successful embassy to the duke of Choiseul and in 1770 was nominated to the Council of Two Hundred. His travels widened Deluc’s knowledge of landscape, but most of his early writings on natural science were based on numerous excursions to the Alps and the Jura. As was then fashionable, he gradually amassed, with the help of his brother, Guillaume Antoine, a collection of minerals and of flora and fauna. Later his nephew, Jean André Deluc, expanded this collection and took on his uncle’s role of voluminous discourser on geological topics, trying, for example, to dissuade Buckland and Murchison from accepting any theory regarding glacial action.

Deluc’s commercial affairs failed in 1773 and he left Geneva, returning only once, for a few days. However, his decision to migrate to England afforded him greater opportunity for carrying out scientific research and writing, which he did for another fortyfour years. In London, soon after his arrival, he was made a fellow of the Royal Society and appointed reader to Queen Charlotte, a post with an income adequate to allow him ample leisure. During this period of his life Deluc undertook several tours on the Continent and lived for six years (1798–1804) in Germany, where he was a nonparticipant honorary professor of philosophy and geology at Göttingen University. He was also a correspondent of the Paris Academy of Sciences and a member of several other scientific associations.

Deluc’s favorite fields were geology, meteorology, and natural philosophy or theology, as one might expect of a Calvinistic Genevan who made many scientific excursions to the Alps. By nature an inveterate discourser, he would write in a moderate tone on anything, including, for example, the history of the solar system before the birth of the sun. His great aim was to reconcile Genesis and geology; and his orthodoxy, versatility, prolixity, productivity, high social standing, and facility in languages earned him an exalted contemporary position. Georges Cuvier ranked him among the first geologists of his age, whereas Zittel (p. 77) affirms that although Deluc was “held in high respect and favour during his lifetime, his papers have no permanent place in [the] literature.”

Deluc believed that the six days of the Creation were six epochs that preceded the present state of the globe, which began when cavities in the interior of the earth collapsed and lowered the sea level, thereby exposing the continents. There was thus a distinction between an older creative, or antediluvian, period and a newer, or diluvian, period. Of the former there survived only a few primordial islands, which accounted for the fossils of large animals and the continuity and antiquity of organic life. In the latter period, which started about 2200 b.c., new geological processes were operative but were so ineffectual or incidental that the landscape remained unchanged. To Deluc mountains were the remnants left upstanding when the adjacent areas had collapsed catastrophically, and the large boulders known today as glacial erratics had been blown out when great interior caverns filled with some expansible fluid had collapsed.

In 1790–1791 and later, in many letters, Deluc opposed Hutton’s ideas on present erosion, asserting, for example, that soil is not eroded because if it were there would be none left. In his Elementary Treatise on Geology (1809) he claimed rather bombastically that he could now demonstrate “the conformity of geological monuments with the sublime account of that series of the operations which took place during the Six days, or periods of time, recorded by the inspired penman.” This discursive volume contains, inter alia, four of his earlier letters refuting the ideas of Hutton and Playfair. In his later geological writings Deluc occasionally proffers an astute minor observation but rarely, if ever, is the originator of a new idea.

Deluc’s meteorological researches were of more lasting value but were also hyperbolized by his contemporaries. He is said to have “discovered many facts of considerable importance” relating to atmospheric heat and moisture, but most of his observations had already been developed further by others. For instance, Deluc noticed the disappearance of some heat during the thawing of ice at a time when Joseph Black had already progressed to a hypothesis of latent heat. Deluc, however, probably can claim to be the originator of the theory, later proved more clearly by John Dalton, that the amount of water vapor contained in any space is independent of the density of the air or any other gaseous substance in which it is diffused.

Deluc’s early meteorological interest was mainly in measuring heights by barometer, for which he published improved rules (Philosophical Transactions [1771], 158) based on many experiments with hygrometers, thermometers, and barometers, and particularly on the fall in the boiling point of water with diminishing atmospheric pressure and increasing altitude. He devised a hygrometer similar to a mercury thermometer but with an ivory bulb that expanded when moistened and thus caused the mercury to descend (Philosophical Transactions [1773], 404). Humboldt compared the merits of this with Saussure’s hair hygrometer: the latter proved better for measuring altitude on mountains and the former for use at sea level, but Deluc’s hygrometer worked so slowly that its readings could seldom be combined with those of other instruments.

Deluc’s influence on popular early nineteenth-century British meteorology texts was considerable. J. F. Daniell, in his Meteorological Essays and Observations, based his account of atmospheric evaporation and condensation largely on extracts from “the works of Deluc, who was probably one of the most accurate observers of nature that ever existed, and who seldom, indeed, allowed any hypothetical considerations to warp his description of what he observed” (2nd ed. [1827], p. 506). This hyperbole stemmed from Deluc’s visual observations on clouds and ground (radiation) fog, which, he stated, can be seen to change shape and evaporate at the same time that they are forming.

The barometric controversy between H. B. de Saussure, professor of philosophy at Geneva, and Deluc is one of lasting scientific interest. In Essais sur l’hygromètre (1783, p. 282) Saussure stated that some of Deluc’s findings were based on specious reasoning and inadequate experimentation: “Mr. Deluc supposes that pure air is heavier than air mixed with water vapor.... This supposition explains well why a lowering of the barometer is a sign of rain....” Saussure, experimenting with closed containers, had found little difference in weight between dry air and humid air, and considered the differences quite inadequate to explain the large variations in barometric pressure that occurred at ground level in Europe. Modern meteorology has proved that Deluc was right, whereas Saussure was groping toward the influence of air masses and of the passage of cyclonic depressions and anticyclones.

The significance of Deluc’s contributions to physics is greatly exaggerated. In 1809 he sent a long article to the Royal Society discussing the mode of action of the galvanic pile and showing that “in Volta’s pile, the chemical effects can be separated from the electrical.” This, as a biographer in Philosophical Magazine (50 , 393–394) wrote, “...led that ingenious philosopher [Deluc] to construct a new meteorological instrument, very desirable for acquiring a knowledge of atmospherical phaenomena, and which he called the Electric column.” The ideas expressed differed so much from those prevalent in London that the council of the Royal Society “deemed it inexpedient to admit them into the Transactions, “and the article was also published in Nicholson’s Journal (26 [1810]). This “electric column” (or electroscope) consisted of numerous disks of zinc foil and of paper silvered on one side only, piled horizontally in order of zinc, silver, and paper within a glass tube and firmly screwed together. When the uppermost silver was connected by a wire with the lowest zinc disk, an electric current passed along the wire. Today, however, it is hard to see the importance to meteorology and physics of this electric column, which was later improved by Giuseppe Zamboni. It is claimed as a “very valuable discovery” by Deluc’s admirers but its principles, at least, had already been stated clearly by Volta on the Continent and probably also during his visit to England.

Deluc’s other ventures into physics and chemistry showed all too clearly his inability to assess truly the quality of progress at home and abroad. He strenuously opposed the new chemical theory associated with Lavoisier and attempted to show in two memoirs on that theory, prefixed to his Introduction à la physique terrestre par les fluides expansibles, that meteorological phenomena strongly militate against it and in general that the hypothesis of the composition of water (the fundamental point in the theory) has maintained itself only by numerous other hypotheses which are in contradiction with known facts.

BIBLIOGRAPHY

I. Original Works. Deluc wrote numerous long articles for periodicals, the chief being Philosophical Transactions of the Royal Society; The Philosophical Magazine and Journal; British Critic, especially 1793–1795, for letters addressed to J. F. Blumenbach; Monthly Magazine; Monthly Review, especially 1790 and 1791, for letters to Hutton; and Nicholson’s Journal. Many of these letters or articles were republished later in the following books: Recherches sur les modifications de l’atmosphère, 2 vols. (Geneva, 1772), 4 vols. (Paris, 1784); Lettres physiques et morales sur les montagnes et sur l’histoire de la terre et de l’homme. (Adressées à la reine de la Grande Bretagne), 5 vols. (The Hague, 1779); Idées sur la météorologie, 2 vols. (Paris, 1786); Lettres sur l’histoire physique de la terre (Paris, 1798) abridged trans. by Henry de la Fite (London, 1831); Lettres sur l’education religieuse de l’enfance (Berlin, 1799); Bacon tel qu’il est (Berlin, 1800); Lettres sur le christianisme adressées à M. le pasteur Teller (Berlin-Hannover, 1801; 1803); Précis de la philosophie de Bacon, 2 vols. (Paris, 1802); Introduction à la physique terrestre par les fluides expansibles, 2 vols. (Paris, 1803); Traité élémentaire sur le fluide électricogalvanique, 2 vols. (Paris, 1804); Traité él’mentaire de géologie (Paris, 1809), trans. by Henry de la Fite (London, 1809); Geological Travels in the North of Europe and in England, 3 vols. (London, 1810–1811); Geological Travels in Some Parts of France, Switzerland and Germany (London, 1813).

II. Secondary Literature. On Deluc or his work, see R. J. Chorley, A. J. Dunn, and R. P. Beckinsale, History of the Study of Landforms, I (London, 1964), passim; Encyclopaedia Britannica, 11th ed. (1910–1911); a biography in Gentleman’s Magazine (1817), pt. 2, 629; Charles C. Gillispie, Genesis and Geology (Cambridge-New York, 1951; 1959); passim; W. J. Harrison, in Dictionary of National Biography, XIV (1888), 328–329; C. Lyell, Principles of Geology, 12th ed. (London, 1875), I, 80; II, 506, 507; a biography in The Philosophical Magazine and Journal, 50 , no. 1 (Nov. 1817), 393–394; and K. A. von Zittel, History of Geology and Palaeontology, M. M. Ogilvie-Gordon, trans. (London, 1901), which mentions Deluc’s theoretical articles in Journal de physique—this otherwise excellent assessment wrongly suggests that Deluc first proposed the term “geology” in its modern sense.

Robert P. Beckinsale