# Encyclopædia Britannica, Ninth Edition/Jean André Deluc

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​DELUC, Jean André (1727–1817), geologist and meteorologist, born at Geneva, Februarys, 1727, was descended from a family which had emigrated from Lucca and settled at Geneva in the 15th century. His father, Franois Deluc, was the author of some publications in refuta tion of Mandeville and other rationalistic writers, which are best known through Rousseau s humorous account of his ennui in reading them ; and he gave his son an excellent education, chiefly in mathematics and natural science. On completing it he engaged in commerce, which principally occupied the first forty-six years of his life, without any other interruption than that which was occasioned by some journeys of business into the neigh bouring countries, and a few scientific excursions among the Alps. During these, however, he collected by degrees, in conjunction with his brother Guillaume Antoine, a splendid museum of mineralogy and of natural histoiy in general, which was afterwards increased by his nephew Andre" Deluc. He at the same time took a prominent part in politics. In 1768 he was sent to Paris on an embassy to the Due de Choiseul, whose friendship he succeeded in gaining. In 1770 he was nominated one of the Council of Two Hundred. Three years later unexpected reverses in business made it advisable for him to quit his native town, which he only revisited once for a few days. The change was welcome in so far as it set him entirely free for scientific pursuits, and it was with little regret that he removed to England in 1773. He was made a fellow of the Royal Society in the same year, and received the appointment of reader to Queen Charlotte, which he con tinued to hold for forty-four years, and which afforded him both leisure and a competent income. In the latter part of his life he obtained leave to make several tours in Switzerland, France, Holland, and Germany. In Germany he passed the six years from 1798 to 1804 ; and after his return he undertook a geological tour through England. When he was at GSttingen, in the beginning of his German tour, he received the compliment of being appointed honorary professor of geology in that university ; but he never entered upon the active duties of a professorship. He was also a correspondent of the Academy of Sciences at Paris, and a member of several other scientific associa tions. His favourite studies were geology and meteorology. The situation of his native country had naturally led him to contemplate the peculiarities of the earth s structure, and the properties of the atmosphere, as particularly dis played in mountainous countries, and as subservient to tho measurement of heights. He inherited from his father a sincere veneration for the doctrines of Christianity, and a disposition to defend the Mosaic account of the creation against the criticism whose principal weapons were furnished by his favourite science. His royal patroness was most anxious to encourage and promote his labours in this field ; and he was generally supposed to have had ​great success in removing the objections which had been advanced by his antagonists against the comparatively recent formation of t-ho present continents. According to Cuvier, he ranked among the first geologists of his age. His principal geological work, Lettres physiques et morales rur rhistoire de la terre (6 vols. 8vo, The Hague, 1778), was dedicated to Queen Charlotte. It dealt with the appearance of mountains and the antiquity of the human race, explained the six clays of the Mosaic creation as so many epochs preceding the actual state of the globe, and attributes the deluge to the filling up o.f cavities supposed to have been left void in the interior of the earth. This attempt to reconcile religion and science, so often since repeated, was ingenious and for a time successful with most minds. The theory of the Mosaic days was maintained in one form or other by several later geologists of high repute, though it is scarcely now thought worth discussion by any to whom that title can justly be applied. Deluc s original experiments relating to meteorology are more valuable to the natural philosopher than most of his geological work; and he .discovered many facts of consider able importance relating to heat and moisture. He noticed the disappearance of heat in the thawing of ice about the same time that Black founded on it his ingenious hypothesis of latent heat. He ascertained that water was more dense about 40 Fahr. than at the temperature of freezing, expanding equally on each side of the maximum ; and he was the originator of the theory afterward re-advanced by Dalton, that the quantity of aqueous vapour contained in any space is independent of the presence or density of the air, or of any other elastic fluid ; though it appears difficult to reconcile this opinion with some of the experi ments of Deluc s great rival, Saussure, a philosopher who, as he very candidly allows, made in many respects more rapid progress in hygrometry than himself. Deluc s comparative experiments on his own hygrometer and on Saussure s show only that both are imperfect ; but it may be inferred from them that a mean between the two would in general approach much nearer to the natural scale than either taken separately. It appears also probable that Saussure s is rather less injured by time than Deluc s, which has been found to indicate an increasing amount of mean moisture every year. Deluc was a man of warm feelings, and of gentle and obliging manners, and his literary and scientific merits, as well as his unremitting attention to the service of the queen, insured her respect and kindness. He saw her daily for many years, and in his last illness, which was long and painful, she showed him repeated marks of benevolent regard. He died at Windsor on the 7th of November 1817. A brief notice of his more important works, in addition to that mentioned above, will give a clear idea of the nature and range of his scientific activity. His Recherches sur Us modifications de T Atmosphere (2 vols. 4to, Geneva, 1772 ; 4 vols. 8vo, Par. 1784), contains many accurate and ingenious experiments upon moisture, evaporation, and the indications of hygrometers and thermometers, applied to the barometer employed in determining heights. In the Phil. Trans., 1773, appeared his account of a new hygrometer, which resembled a mercurial thermometer, with an ivory bulb, which expanded by moisture, and caused the mercury to descend. The first correct rules ever published for measuring heights by the barometer were those he gave in the Phil. Trans., 1771, p. 158. His Lettres sur VHistoire physique de la Terre (8vo, Par. 1798) were addressed to Professor Blumenbach. The substance had already appeared in the Journal de Physique, for 1790, 1791, and 1798. The volume contains an essay written for a prize at Haarlem in 1791, but without success, on the existence of a General Principle of Morality. It also gives an interesting account of some conversations of the author with Voltaire and Rousseau, Deluc was an ardent admirer of Bacon, on whose writings he published two works, Bacon tel qu il est (8vo, Berlin, 1800), shewing the bad faith of the French translator, who had omitted many passages favourable to revealed religion, and Precis de la Philosophic de Bacon (2 vols. 8vo, Paris, 1802), giving an in. teresting view of the progress of natural science. Lettres sur It Christianisjne (Berlin and Hanover, 1801, 1803) was a contro versial correspondence with Dr Teller of Berlin in regard to the Mosaic cosmogony. His Traite elementaire de Geologic (8vo, Paris, 1809, also in English, by Delafite, the same year), was principally intended as a refutation of the Vulcanian system of Hutton and Playfair, who deduced the changes of the earth s structure from the operation of fire, and attributed a higher antiquity to the present state of the continents than is required in the Neptunian system adopted by Deluc after Dolomieu. He sent to the Royal Society, in 1809, a long paper on separating the chemical from the elec trical effect of the pile, with a description of the electric column and aerial electroscope, in which he advanced opinions so little in unison with the latest discoveries of the day, that the council deemed it inexpedient to admit them into the Transactions. He had, indeed, on other occasions shown somewhat too much scepticism in the rejection of new facts ; and he had never been convinced even of Cavendish s all-important discovery of the composition of water. The paper was afterwards published in Nicholson s Journal (xxvi.), and the dry column described in it was constructed by various experimental philosophers. Many other of his papers on subjects kindred to those already men tioned are to be found in the Transactions and in the Philo sophical Magazine. See Philosophical Magazine, November 1817.