Le Roy, Charles

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Le Roy, Charles

(b. Paris, France, 12 January 1726; d. Paris, 10 December 1779),

physics, meteorology, medicine.

Le Roy was a member of a distinguished family. His father, Julien, was a celebrated horologist. One of his older brothers, Jean-Baptiste, was an eminent physicist and a member of the Paris Academy of Sciences, and his younger brother, Julien-David, was a noted architect and historian of architecture. Charles received his medical education at the University of Montpellier. After traveling extensively in Italy, he returned to Montpellier as a member of the Faculty of Medicine.

Le Roy was a member of the Royal Academy of Sciences at Montpellier and a corresponding member of the Paris Academy of Sciences. In 1777 he returned to Paris, where he practiced medicine until his death in 1779.

Although a physician and teacher of medicine, Le Roy made his most important scientific contribution in physics. In 1751 he published a theory of evaporation that was to dominate discussions of the topic for more than forty years.

In the early eighteenth century, theories of evaporation were usually couched in the idiom of the prevailing mechanical philosophy. Many theories ascribed the “elevation” of vapors in the air to the specific legerity of water particles. Some scientists believed that the molecules of a liquid could be expanded by heat until their density became less than that of the atmosphere, at which point they would rise into the air rather like vacuum-filled balloons. Followers of Descartes thought that during evaporation “subtile matter” infiltrated particles of a liquid, thus reducing their specific gravity and causing them to rise into the ambient air. Other theories, especially those formulated by the followers of Newton, depended upon the concept of attractive forces acting at a distance. Desaguliers, for example, thought that evaporation was caused by atmospheric electricity attracting water particles into the air.

The problem with these early mechanical theories was that they were based upon untestable hypotheses concerning the behavior of submicroscopic particles. To invent an explanation of this kind, it was necessary only to conceive a plausible mechanism and present it as fact. To be sure, such mechanical conceptions could not be disproved—but they could not be proved either.

The value of Le Roy’s theory was in its radically different approach to the problem of evaporation. Rather than attempting to deduce an explanation from an imagined account of the behavior of unseen particles, he presented instead a simple analogy between observable phenomena. Vapors, he said, are to the air as dissolved salts are to water. Le Roy made no attempt to explain the mechanism of solution itself, which he considered beyond the capabilities of the science of his day. It was sufficient merely to show that the phenomena of solution and evaporation were analogous and thus avoid useless debate over alternative mechanical hypotheses. All empirical generalizations concerning evaporation and vapors could be accounted for by referring to the analogous generalizations concerning the behavior of solution. For example, the fact that improved circulation and constant renewing of the air increases the rate of evaporation is analogous to the fact that stirring a liquid increases the rate at which salts dissolve in it. Heating will also increase the rates of both evaporation and dissolution, whereas cooling will retard both processes. Furthermore, just as a liquid may be saturated with a salt, so too may the atmosphere be saturated with water; and just as cooling a saturated solution will cause precipitation of the dissolved salt, so too will cooling a saturated atmosphere cause precipitation of water. In fact, the use of the word “precipitation”in meteorological contexts seems to be a linguistic remnant of Le Roy’s once-popular theory.

The major weakness of Le Roy’ s solution theory of vapors was its inability to explain convincingly the widely known phenomenon of evaporation in Vacuo. Clearly, liquids could not dissolve themselves in mere nothing. Le Roy’s attempted explanation of this anomaly was farfetched. He knew from Stephen Hales’s famous experiments that water contained a large quantity of dissolved air. Le Roy supposed that when water was placed in a vacuum apparatus, the air came out of it and dissolved the very water in which it had itself previously been dissolved.

There is nothing to indicate that Le Roy undertook any but the most casual experiments to substantiate his theory. He seems to have derived most of his empirical generalizations from everyday observation and from the writings of others. Indeed, the theory itself was probably anticipated by other scientists, some of whom Le Roy mentioned in his articles. Le Roy’ s version of the solution theory was, however, the most clearly and completely developed, and through its publication in the Encyclopédie it became widely known and accepted despite its one rather obvious weakness. Other versions of the theory were subsequently published in Britain by Benjamin Franklin and Henry Home, Lord Kames.

Although the solution theory of vapors has proved untenable, Le Roy nevertheless performed an invaluable service to science. By clearing away the arcane mechanisms that had been conceived by his predecessors and presenting instead a clear, testable hypothesis, he prepared the way for the important advancements in the theory of evaporation made by Lavoisier and Dalton. In particular, by focusing on the anomaly of evaporation in Vacuo, he directed attention to a phenomenon that was to prove a key to the understanding of the gaseous state. In his insistence on the use of analogy instead of mechanism, he anticipated a mode of scientific explanation that was to prove remarkably fruitful in the hands of more thoughtful experimentalists, and notably in Lavoisier’s. Finally, the theory itself was, for its time, a good one. It proved useful in most instances and provided an acceptable conceptual framework for dealing with many phenomena in the realms of meteorology, physics, and chemistry. The theory also provided a criterion for distinguishing true vapors from smokes and exhalations: true vapors, like true solutions, are perfectly transparent; whereas smokes and exhalations, like liquid suspensions, are cloudy.

In addition to his work on evaporation, Le Roy also communicated to the Academy a number of other observations. While traveling in Italy, he experimented on the asphyxiating gas produced in the Grotto del Cane, near Naples, and he attempted to provide an explanation for marine phosphorescence in the Mediterranean Sea. During his tenure at the Faculty of Medicine at Montpellier, he wrote extensive treatises on mineral waters, physiology, medicine, and chemistry.


I. Original Works. Le Roy’s principal works are “Mémoire sur l’élévation et la suspension de l’eau dans l’air,” in Histoire et mémoires de l’Académie Royale des Sciences (1751), pp.481-518; “Évaporation,” In Encyclopédie, VI (Paris, 1756), pp. 123-130; and Mélanges de physique et de médecine (paris, 1771).

II. Secondary Literature. For biographical references, see the éloges cited in Nouvelle biographie générale, XXX (Paris, 1862), 892; and L. Dulieu, “Un Parisien professeur à l’Université de Medecine de Montpellier: Charles Le Roy (1726-1779),” in Revue d’histoire des sciences et leurs applications, 6(1953), 50-59. On eighteenth-century theories of evaporation, see S. A. Dyment, “Some Eighteenth Century Ideas Concerning Aqueous Vapor and Evaporation,” In Annals of Science, 2 (1937), 465-473; and J. B. Gough, “Nouvelle contribution à l’évolution des idées de Lavoisier sur la nature de l’air et sur la calcination des métaux,” in Archives internationales d’histories des sciences,22, no. 89 (July-Dec. 1969), 267-275. On the solution theory, see Benjamin Franklin, “physical and Meteorological Observations, Conjectures, and Suppositions,” in Letters and Papers on Philosophical Subjects (London, 1769); and Lord Kames’s article on evaporation in Essays and Observations, Physical and Literary, Read Before a Society in Edinburgh, III (Edinburgh, 1772), pp. 80 ff.

J. B. Gough