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... L A P L A C E ? Pierre Simon Marquis de Laplace ( ), French astronomer and mathematician, best known for his successful application of Newton's theory of gravitation to account for all planetary motion in the solar system. Laplace was born in Normandy and educated at the military school in Beaumont. In 1767 he became professor of mathematics at the École Militaire in Paris and in 1785 he was elected to the Academy of Sciences. Laplace's great work was done in developing the mathematical analysis of the system of gravitational astronomy worked out by Sir Isaac Newton. Laplace proved that planetary motions are stable, and that perturbations produced by mutual influence of planets or by external bodies, such as comets, are only temporary. He attempted to give a rational theory of the origin of the solar system in his nebular hypothesis of stellar evolution. In Traité de mécanique céleste (Treatise on Celestial Mechanics, 5 volumes, ) Laplace systematized all the mathematical work that had been done on gravitation. Exposition du système du monde (Explanation of the World System, 1796) contained a summary of the history of astronomy. He also worked on the theory of probabilities in his Théorie analytique des probabilités (Analytical Theory of Probabilities, 1812) and Philosophical Essay on Probabilities (1814; trans. 1905).

... L A V O I S I E R ? Antoine Laurent Lavoisier ( ), was a French chemist, who is considered the founder of modern chemistry. Lavoisier was born on August 26, 1743, in Paris and was educated at the Collège Mazarin. He was elected a member of the Academy of Sciences in Lavoisier showed that, although matter changes its state in a chemical reaction, the quantity of matter is the same at the end as at the beginning of every chemical reaction. These experiments provided evidence for the law of the conservation of matter. Lavoisier also investigated the composition of water, and he named the components of water oxygen and hydrogen. Some of Lavoisier's most important experiments examined the nature of combustion, or burning. Through these experiments, he demonstrated that burning is a process that involves the combination of a substance with oxygen. He also demonstrated the role of oxygen in animal and plant respiration. Lavoisier's explanation of combustion replaced the phlogiston theory, which postulates that materials release a substance called phlogiston when they burn.

With the French chemist Claude Louis Berthollet and others, Lavoisier devised a chemical nomenclature, or a system of names, which serves as the basis of the modern system. He described it in Méthode de nomenclature chimique (Method of Chemical Nomenclature, 1787). In Traité élémentaire de chimie (Treatise on Chemical Elements, 1789), Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis, and he devised a theory of the formation of chemical compounds from elements. He also wrote Sur la combustion en general (On Combustion, 1777) and Considerations sur la nature des acides (Considerations on the Nature of Acids, 1778). As one of the farmers-general, he was arrested and tried by the With the French chemist Claude Louis Berthollet and others, Lavoisier devised a chemical nomenclature, or a system of names, which serves as the basis of the modern system. He described it in Méthode de nomenclature chimique (Method of Chemical Nomenclature, 1787). In Traité élémentaire de chimie (Treatise on Chemical Elements, 1789), Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis, and he devised a theory of the formation of chemical compounds from elements. He also wrote Sur la combustion en general (On Combustion, 1777) and Considerations sur la nature des acides (Considerations on the Nature of Acids, 1778). With the French chemist Claude Louis Berthollet and others, Lavoisier devised a chemical nomenclature, or a system of names, which serves as the basis of the modern system. He described it in Méthode de nomenclature chimique (Method of Chemical Nomenclature, 1787). In Traité élémentaire de chimie (Treatise on Chemical Elements, 1789), Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis, and he devised a theory of the formation of chemical compounds from elements. He also wrote Sur la combustion en general (On Combustion, 1777) and Considerations sur la nature des acides (Considerations on the Nature of Acids, 1778). As one of the farmers-general, he was arrested and tried by the revolutionary tribunal, and guillotined on May 8, 1794.

... L E I B N I T Z ? Baron Gottfried Wilhelm von Leibniz ( ), German philosopher, mathematician, and statesman, regarded as one of the supreme intellects of the 17th century. Leibniz was born in Leipzig. He was educated at the universities of Leipzig, Jena, and Altdorf. Beginning in 1666, the year in which he was awarded a doctorate in law, he served Johann Philipp von Schönborn, archbishop elector of Mainz, in a variety of legal, political, and diplomatic capacities. In 1673, when the elector's reign ended, Leibniz went to Paris. He remained there for three years and also visited Amsterdam and London, devoting his time to the study of mathematics, science, and philosophy. In 1676 he was appointed librarian and privy councillor at the court of Hannover. For the 40 years until his death, he served Ernest Augustus, duke of Brunswick-Lüneburg, later elector of Hannover, and George Louis, elector of Hannover, later George I, king of Great Britain and Ireland.

Leibniz was considered a universal genius by his contemporaries. His work encompasses not only mathematics and philosophy but also theology, law, diplomacy, politics, history, philology, and physics. Leibniz's contribution in mathematics was to discover, in 1675, the fundamental principles of infinitesimal calculus. This discovery was arrived at independently of the discoveries of the English scientist Sir Isaac Newton, whose system of calculus was invented in Leibniz's system was published in 1684, Newton's in 1687, and the method of notation devised by Leibniz was universally adopted. In 1672 he also invented a calculating machine capable of multiplying, dividing, and extracting square roots, and he is considered a pioneer in the development of mathematical logic. Leibniz's contribution in mathematics was to discover, in 1675, the fundamental principles of infinitesimal calculus. This discovery was arrived at independently of the discoveries of the English scientist Sir Isaac Newton, whose system of calculus was invented in Leibniz's system was published in 1684, Newton's in 1687, and the method of notation devised by Leibniz was universally adopted. In 1672 he also invented a calculating machine capable of multiplying, dividing, and extracting square roots, and he is considered a pioneer in the development of mathematical logic.

... L O R E N T Z ? Hendrik Antoon Lorentz ( ), Dutch physicist and Nobel laureate. Lorentz was born in Arnhem and educated at the Leiden University, where he became professor of mathematical physics in He developed the electromagnetic theory of light and the electron theory of matter and formulated a consistent theory of electricity, magnetism, and light. With the Irish physicist George Francis FitzGerald, he formulated a theory on the change in shape of a body resulting from its motion; the effect, known as the Lorentz-FitzGerald contraction, was one of several important contributions that Lorentz made to the development of the theory of relativity. For his explanation of the phenomenon known as the Zeeman effect, Lorentz shared the 1902 Nobel Prize in physics with the Dutch physicist Pieter Zeeman..