LUC (JOHN ANDREW DE), a natural philosopher of great merit and celebrity, born at Geneva, 8th February 1727, was the son of James Francis de Luc, descended from a family who had emigrated from Lucca, and settled at Geneva, in the fifteenth century.
His father was the author of some very respectable publications in refutation of Mandeville, and other sceptical writers; and he had the means of giving his son an excellent education, although he found it convenient to establish him in a commercial engagement, 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 neighbouring countries, and a few scientific excursions among the Alps: during these, however, he collected by degrees, in conjunction with his brother William Antony, a splendid museum of mineralogy and of natural history in general, which is still preserved at Geneva by the younger Deluc, his nephew. He took, at the same time, his share of the public business of the state, as one of the council of 200, and he is still remembered with respect by his fellow citizens, though he revisited them but once, and that for a few days only, after his emigration, which was the consequence of some unexpected misfortunes in commerce. He bore them with fortitude, and he rather rejoiced than lamented at the change of his pursuits, when he removed to England in 1773. He was made a Fellow of the Royal Society in the same year, and was appointed Reader to the Queen; a situation which he continued 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 perform several tours in Switzerland, France, Holland, and Germany; in this last country he passed six years, from 1798 to 1804; and after his return he undertook a Geological Tour through England. When he was at Gottingen, 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 on 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 associations.
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 displayed in mountainous countries, and as subservient to the 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 date of the creation, against the incredulity of the age. His royal patroness was most anxious to encourage and promote his labours in this field; and he is universally allowed to have had great success in removing the specious objections which had been advanced by his antagonists, against the comparatively recent formation of the present continents. The testimony of Cuvier is sufficient to establish his character in this capacity, and to place him, at the same time, in the first rank of modern geologists. His original experiments, relating to meteorology, are, however, not less valuable to the natural philosopher; and he discovered many facts of considerable 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° of Fahrenheit than at the temperature of freezing, expanding equally on each side of the maximum: and he was the original author of the opinion lately readvanced by Mr 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 experiments of our author'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 both 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 a greater degree of mean moisture every year than the last.
He was a man of warm feelings, and of gentle and obliging manners, fulfilling on all occasions the various duties of a husband, a father, a master, and a friend; at the same time, that his literary and scientific merits, and his unremitting attention to
the service of the Queen, ensured her respect and her 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, leaving a variety of works, which will long be remembered in the scientific world.
1. Recherches sur les modifications de l'atmosphère, 2 v. 4. Geneva, 1772. 4 v. 8. Par. 1784. Containing many accurate and ingenious experiments on moisture, evaporation, and the indications of hygrometers and thermometers, applied to the employment of the barometer in the measurement of the height of mountains.
2. Relation de différents voyages dans les Alpes de Faucigny, 12. Maastricht, 1771. Written principally by Dentand, who accompanied the two Delucs in these expeditions.
3. Account of a new hygrometer. Phil. Trans. 1773, p. 404. Like a mercurial thermometer, with an ivory bulb, which expanded by moisture, and caused the mercury to descend.
4. Rules for measuring heights by the barometer. Phil. Trans. 1771, p. 158. The first correct rules that had been made public.
5. Barometrical observations on the depth of the mines in the Harz. Phil. Trans. 1777, p. 401. Examples of the application of the rules.
6. An essay on pyrometry and areometry. Phil. Trans. 1778, p. 419. A paper containing many valuable remarks on physical measures in general.
7. Lettres physiques et morales sur l'histoire de la terre, 6 v. 8. Hague, 1778. Dedicated to the Queen; relating particularly to the appearances of mountains, and to the antiquity of the human race; explaining the six days of the Mosaic creation as so many periods, preceding the epoch of the actual state of the globe; and attributing the deluge to the filling up of cavities, supposed to have been left void in the interior of the earth. The whole work is intermixed with interesting observations on men and manners.
8. A second paper concerning some barometrical measurements in the mines of the Harz. Phil. Trans. 1779, p. 485.
9. Lettres sur quelques parties de la Suisse. 8. 1781. Also addressed to the Queen.
10. Nouvelles idées sur la météorologie. 2 v. in 3. Lond. 1787. A very valuable collection of observations and experiments, including some original remarks on electricity.
11. Several papers on hygrometry, on vapour, and rain, on meteorology in general, on expansion, and on refraction, in Rozier's Journal de Physique, XXX., XXXII., XXXVI., XXXVII., XLIII.
12. Some letters On the physical history of the earth, in the Monthly Review, enlarged, especially June 1790, and Vol. II. Appendix.
13. On hygrometry. Phil. Trans. 1791, p. 1, 389. In one of these very important papers, the whalebone hygrometer is described.
14. On evaporation. Phil. Trans. 1792, p. 400. Among the fundamental principles laid down in this paper, the independence of vapour and air is asserted.
15. Lettres sur l'histoire physique de la terre. 8.
Par. 1798. Addressed to Professor Blumenbach, and published by Mr Emery, a clergyman at Paris. The substance had already appeared in the Journal de Physique for 1790, 1791, and 1798. We find in this volume an essay written for a prize at Haarlem, in 1791, but without success, On the existence of a general principle of morality. It contains an interesting account of some conversations of the author with Voltaire and Rousseau.
16. Lettres sur l'éducation religieuse de l'enfance, 8. Berlin, 1799.
17. Bacon tel qu'il est. 8. Berlin, 1800. Showing the bad faith of the French translator, who had omitted many passages favourable to revealed religion.
18. Précis de la philosophie de Bacon. 2 v. 8. Par. 1802. Giving an interesting view of the progress of natural science.
19. Lettres sur le Christianisme, Berlin and Hanover, 1801, 1803. A correspondence with Mr Teller.
20. Introduction à la physique terrestre par les fluides expansibles. 8. Par. 1803.
21. Traité élémentaire sur le fluide galvanique. 8. Paris, 1804.
22. A paper on lavas. Journal des Mines, exv. Nicholson, XX.
23, 24. Several articles in the British Critic, and in the Monthly Magazine.
25. Traité élémentaire de géologie. 8. Paris, 1809. Also in English, by Delafite, the same year. This volume is less strictly introductory to geology than the Lettres sur la terre. It is 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.
26. He sent to the Royal Society, in 1809, a long paper On separating the chemical from the electrical effects 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, especially with those of the present President of the Society, that the council probably thought it would be either encouraging error or leading to controversy to admit them into the Transactions. He had, indeed, on other occasions, shown somewhat too much scepticism in the rejection of new facts; and had never been convinced even of Mr 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. It exhibited a continual vibrating motion, which was made more sensible by the sound of a little bell, struck by the pendulum at each alternation; and the vibration was more or less rapid, according to the state of the atmospheric electricity, and according to other circumstances affecting the column: but the motion ceased at last, after a continuance of several months, or perhaps years. There are also papers in Volumes XXI. XXII. XXVII. XXVIII. XXXII. XXXIII. and
XXXV., mostly On electricity and galvanism, together with one on hygrology, a Letter to Bode on comets, and a fanciful theory of the origin of the Heat derived from compression; some of them are dated from Ashfield, near Honiton, in Devonshire.
27. In the Philosophical Magazine, Volumes XXXV. XLII. XLIII. and XLV., there are also some papers On electricity and geology, especially on that of St Michael's Mount, of Vesuvius, and of Northumberland; and a note On the sympathetic vibrations of the pendulums of two clocks, placed near each other.
28. Geological travels in the north of Europe. 8. Lond. 1810.
29. Geological travels in England. 2 v. 8. Lond. 1811.
30. Geological travels in Switzerland and Germany. 2 v. 8. Lond. 1813.
31. An "Abridgment of geology, published in 1817," when he was in his ninetieth year, is mentioned as one of his best works; but it seems to have been only a republication or a translation of some former treatise, perhaps the Traité élémentaire.
[Philosophical Magazine, Nov. 1817. MONOD and WEISS, in Biographie Universelle, XXV. 8. Paris, 1820.] (o. n.)
LUCIMETER.—This name has been given to an apparatus employed by Bouguer for measuring the intensity of the light which proceeds from different bodies.
In the case of two lamps or candles burning near to each other, the intensity of the light may be compared, by comparing the intensity of the shadows of an adjacent body. The shadow produced by interposing the body in the stronger light is darker than its shadow, formed by the interposition of the body in the rays of the weaker; because the first shadow is enlightened only by the light it receives from the weaker light, whilst the other shadow is more strongly enlightened by the light from the stronger light. But this method will not serve for comparing the intensity of the light of the sun and moon, for the light of the sun so greatly exceeds the other, that when both sun and moon are visible at the same time, the light of the moon is far too weak for any shadow to be formed by intercepting it.
For the purpose of measuring and comparing the intensity of the light of the sun and moon, Bouguer reduced the light of the sun and of the moon till the light appeared to be equal to that of a candle. The eye is able to judge whether or not two lights are equal; but if the lights are unequal, it is impossible to estimate by inspection how much the one exceeds the other. Being able to calculate how much he had reduced the light of the sun and of the moon, he was enabled to say what proportion their light bore to that of the candle.
For the former, Bouguer, when the sun was at an elevation of 31 degrees above the horizon, received the light into a dark room, through a hole of of an inch in diameter. In this hole was placed a concave glass to weaken the light, by making the rays diverge more than they would in passing through the hole without a concave glass. He received the image formed by the divergent rays at a distance of five or six feet, by interposing a screen, which formed
a right section of the cone of divergent rays: this right section was a round image of the sun, of of an inch in diameter; therefore the light was diminished by the square of that number 11,664, since the light received through a hole of one line in diameter was diffused over a circle of 108 lines in diameter, and of 1164 times the area. This light he found equal to that of a candle placed at a distance of 16 feet.
He employed the same concave glass to receive the light of the full moon; the moon being nearly at its mean distance from the earth, and at 31 degrees of altitude; the same altitude as the sun had in his observation of that luminary. The light of the moon being weak, he received the image at of an inch from the concave glass, and then the light of the image was so weak as to be equal to the light of a candle at the distance of 50 feet.
Now the light of the moon was diminished in the proportion of the square of one to sixty-four, which is the square of the diameter of its circular image. If the light of the moon had been diminished 11,664 times, it would have been equal to the light of the candle removed to a distance of 675 feet; for as 8, the diameter of the image, is to 50, the distance, so is 108 the diameter of the image when the light is reduced 11,664 times, to 675 feet, the distance of the candle, which would produce that degree of light. Therefore the light of the sun being equal to that of a candle placed at the distance of 16 inches, and the light of the moon being equal to the light of the same candle placed at the distance of 675 feet, that is, 8100 inches, it follows that the light of the sun is to the light of the moon as 65,610,000, the square of 8100, is to 256, the square of 16. This experiment gave the light of the sun 256,289 times greater than the light of the full moon. Bouguer had results somewhat different from other experiments, and taking a mean of these results, he concludes that the light of the sun is 300,000 times greater than the light of the full moon. The light of the moon, when collected into a focus by a concave mirror, which condenses the light into a space 306 times less than the natural state of the light, produces no sensible heat.
Bouguer found, by a process similar to that above described, that the light of the full moon, when elevated , was to the light of the full moon elevated above the horizon, as 2500 the square of 50, to 1681 the square of 41. The sun at the harbour of Croisic in Brittany, where Bouguer made his experiments, has the same apparent altitudes of and at the summer and winter solstices; and he concludes that the intensity of the light of the sun at the summer solstice is to the intensity of its light at the winter solstice, in the above mentioned ratio of 2500 to 1681, or about three to two. He found that the light of the moon when near the horizon, and about to set, was 2000 times less than the light of the moon elevated . This difference in the light of heavenly bodies, when at different altitudes, proceeds from the want of transparency in the air; for when the heavenly body is not much elevated above the horizon, the rays proceeding from it have a greater distance of atmosphere to pass through
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than the rays from the same body at a greater altitude; and the rays from a heavenly body in the zenith proceed in the direction of a diameter, and therefore traverse the atmosphere by the shortest path. Bouguer calculates that the mass of air passed through by the rays from a heavenly body at , is equal to a supposed mass of air of the uniform density of the air at the surface of the earth, 4275 toises in thickness; and that the mass of air passed through by the rays from a heavenly body in altitude, is equal to a supposed mass of air of the density of the air at the surface, and of 11,744 toises in thickness.
He concludes that light is diminished , or about , by passing through a mass of air of the uniform density of the air at the surface, and 7469 toises in thickness. The rays from a heavenly body near the horizon are likewise impeded by the terrestrial vapours, which vary in density, and render the intensity of the light of a heavenly body at the horizon various at different times. The same cause renders the quantity of the refraction of the heavenly bodies at the horizon uncertain.
In certain states of the air, the discs of the sun and moon, when at the horizon, appear somewhat elliptic; the apparent vertical diameter being less than the horizontal. This is occasioned by the greater refraction of the air through which the under limb is seen; which refraction elevates the apparent place of the under limb, by a quantity sensibly greater than that quantity of refraction by which the upper limb is raised. This alteration of figure, and the colour of the rising and setting sun and moon, indicate the greater or less quantity of vapour diffused in the air, and are usefully referred to by the seaman and farmer as prognostic indications of the weather.
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The existence of a greater or less quantity of vapour mixed with the air is also indicated by the different degrees of intensity of the blue colour of the sky. Some are of opinion, that this colour depends on the colour of the mass of air which forms the atmosphere; others maintain, that it arises from the darkness of space seen through the interposed atmosphere. The blue colour of the sky increases in intensity from the horizon to the zenith, and is particularly intense when seen from the elevated parts of the Alps, because in that situation there are few terrestrial vapours mixed with the air, and it is the white colour of these vapours which renders the blue less intense in lower situations. For the purpose of estimating and noting its intensity, it is compared with different tints of blue painted on a card, as described under the article CYANOMETER.
See Essai d'optique sur la gradation de la lumière, par Bouguer; and Voyage de Humboldt—Relation historique, Chap. III. p. 251. (y.)