Lesage, George-Louis

(b. Geneva, Switzerland, 13 June 1724; d. Geneva, 9 November 1803),

physics.

Lesage’s father, also named George-Louis, was a distinguished teacher of mathematics and physics as well as a moral philosopher and theologian. Born in Conches, in Normandy, of a notorious Huguenot family, he was exiled to England at the age of eight to escape the religious persecutions in France. He married Anne-Marie Camp in 1722 and settled in Geneva shortly before the birth of his only son.

The elder Lesage instructed his son in the classics and the rudiments of mathematics and science. At the collège in Geneva Lesage studied physics with Jean- Louis Calandrani and mathematics with Gabriel Cramer.

Lesage wanted to become a philosopher; but his father, desiring that he should find more remunerative employment, determined instead that his son should become a physician. Lesage reluctantly attended medical school at Basel. After a brief sojourn in Paris, he returned to Geneva and attempted to take up the practice of medicine. The city authorities intervened—as the son of an immigrant, Lesage was not qualified. At first he tried to raise the money to purchase bourgeois status by entering the prize essay contest of the Paris Academy of Sciences. He failed and thus gave up medicine entirely and became a teacher of mathematics.

Lesage’s reputation derived largely from his efforts to explain mechanistically the cause of gravitational phenomena. Although Newton’s law had finally been accepted by Continental scientists, it still presented serious intellectual difficulties. Every particle of matter in the universe supposedly attracted every other inversely as the squares of the distances separating them and directly as the products of their masses. Many found it inconceivable, however, that lumps of inanimate matter could somehow divine the presence of their neighbors, measure the appropriate distances and masses, and attract each other across the intervening space. The absurdity of such a notion was manifest, especially to Continental scientists imbued with the precepts of the Cartesian mechanical philosophy. Yet Newton’s law of gravity had been verified in innumerable instances and without exception. In the minds of most eighteenth-century scientists, the validity of the Newtonian law was unquestionable; but because of their commitment to the mechanical philosophy, they generally—albeit often tacitly—assumed that some underlying impulsive mechanism was responsible for the so-called Newtonian attraction. Newton himself had attempted to provide mechanical explanations of his law but without much success.

Inspired by Lucretius’ De rerum natura and doubtless also by the pervasive climate of Cartesianism, Lesage set about the formidable task of explaining the Newtonian law of attraction in terms of the mechanical philosophy. His basic system rested upon his conception of “otherworldly particles” (particules ultramondaines), so-called because of their exemption from the law of gravity. These gravitational particles were presumed to be extremely small Lucretian atoms that moved in every direction and at very high velocity. Any isolated mass of ordinary matter (its atoms were presumed to be much larger than those composing the gravitational fluid) would not be moved by the impacts of the gravitational particles since they would impinge on it from all directions at once. (Lesage did allow for slight oscillation caused by temporary imbalance of forces, rather like the Brownian motion of particles in a gas or liquid.) Two masses of ordinary matter, however, would block some of the particles coming from either direction along the lines connecting the parts of one with those of the other. Each mass, in effect, would cast a kind of “gravitational shadow” on the other; and the resulting disequilibrium of force would impel the two bodies together, thus giving the illusion of an attraction between the masses. The greater the distance between the two bodies, the less intense would be the effect of the mutual gravitational shadow. Indeed, Lesage maintained that it would vary inversely as the square of the distance, in accordance with the Newtonian law. Similarly, the larger the mass of the body, the more gravitational particles would be intercepted and the greater would be the disequilibrium of force. To make the impulsive force vary with the mass rather than with the surface area of the body was always the most difficult problem faced by those who attempted to adduce mechanical explanations of the Newtonian law. By making his bodies extremely porous and quite large relative to their constituent atoms, Lesage was able at least to achieve an approximation of the proper relation between mass and gravitational force.

Using this ingenious mechanism, Lesage attempted to explain not only gravitation but also chemical affinity and corporeal cohesion. His system was not widely accepted among his contemporaries. It was too burdened with ad hoc assumptions, and because certain parameters (the size of the various atoms and the mean free path of the particles composing the gravitational fluid) could not be established, his theory was not subject to rigorous mathematical analysis. In short, his theory was not unlike the clever but untestable mechanical hypotheses that had burdened physics in the previous century. As a result Lesage’s work was less criticized than neglected.

Despite this neglect and despite the fact that he published relatively little in his lifetime (although he did, in fact, write a great deal) Lesage acquired a fairly extensive reputation, largely through his correspondence with the great natural philosophers and mathematicians of his age. He was elected a correspondent of the Paris Academy of Sciences on 28 February 1761, and after the reorganization of the Academy became a first-class corresponding member on 12 September 1803.

BIBLIOGRAPHY

The only major work published during Lesage’s lifetime was his Essai de chimie mécanique (Rouen, 1758), the main source for his explanation of Newtonian attraction.

On Lesage’s life and work, see Pierre Prévost, Notice de la vie et des écrits de George-Louis Le Sage de Genève (Geneva, 1805), which contains an extensive and detailed biography, pp. 1–184; a list of his shorter published articles and letters, pp.91–95; a list of his many unpublished large works and an account of their contents, pp. 95–101; “Lucrèce Neutonien,” one of Lesage’s many MS works on gravitation, with a conveniently succinct exposition of his system; and a portion of Lesage’s large correspondence with contemporary scientists, philosophers, and mathematicians, pp. 189–502.

J. B. Gough