Although Laplace was, perhaps, the most important mathematical physicist between Newton and Maxwell and also a considerable public figure during a turbulent period in French history, there is no single account of his life, work and influence with any claim to completeness. After his death his colleagues produced the customary obituaries of which the *éloge* by Fourier before the Académie des Sciences is the best known.1 Laplace's works were collected in two editions,2 the seven volumes of his *Oeuvres* of 1843–47 and the 14 of his *Oeuvres complètes* of 1878–1912. The first worthwhile assessment of his scientific work came in a book of 162 tiny pages by Henri Andoyer3 in 1922. When the *Dictionary of scientific biography* started to appear in the 1970s the entry on Laplace was deferred from its proper volume but he then received an unusually long entry4 in a supplementary volume, vol. 15 (1981). This article was written by the editor, Charles Gillispie, with sections contributed by Robert Fox and Ivor Grattan-Guinness. It was revised slightly and reissued as a separate book5 in 1997. Meanwhile, Roger Hahn had been studying Laplace's work and records in great detail for many years, paying particular attention to the surviving letters and other manuscript documents. Laplace's grand-daughter had married into the Colbert family, and most of the domestic letters were thought to have been lost in a fire at the Château de Mailloc in 1925. Hahn found, however, that many had survived, here and elsewhere, and in 1982 he published a calendar of the correspondence. Some years later, after most of the domestic papers had been sold to a Paris book-dealer, he found that these contained many items of which he had previously been unaware. He arranged for all these papers, old and new, to be bought by the Bancroft Library of the University of California at Berkeley and then issued a revised calendar.6 The result of his lifetime's study of the manuscript and printed records appeared first as a biography in French7 in 2004, and now in an English version, which is the subject of this review.

In his opening acknowledgments Hahn observes that the earlier efforts of Andoyer and of Gillispie and his team had been restricted to Laplace's scientific accomplishments and had paid little attention to his ‘other roles in society, his private thoughts on politics and religion, or his family life’. This biography redresses the balance. Hahn tells in some detail of Laplace's upbringing in rural Normandy, of his education first at a school run by the Benedictines and then at the University of Caen, of his decision to abandon any thought of a career in the Church, and of his move to Paris where he was fortunate to engage the patronage of d'Alembert and so to obtain a teaching post at the École Militaire. From here he embarked on a series of researches in mathematics, celestial mechanics and the theory of probability that were to secure his election to the Académie des Sciences. These subjects and the terrestrial physics that he later pursued, sometimes in conjunction with Lavoisier, Berthollet and younger colleagues, were to form the core of his contribution to science. Broadly speaking he followed d'Alembert's programme of completing and perfecting Newton's scheme of things, but with a preference for analysis rather than Newton's more geometrical methods. His most famous contributions to mathematical astronomy were his resolution of the anomaly of the motions of Jupiter and Saturn and his subsequent demonstration of the stability of the Solar System. A full account of this and other contributions was delayed by the onset of the Revolution in 1789 but appeared in the five volumes of his *Traité de mécanique céleste* from 1798 to 1825. Hahn gives us a full account of what Laplace accomplished but without any of the technical detail of how he did it. For that the reader has to go to the account by Gillispie and his colleagues, because Hahn's book has no diagrams or equations.

The Revolution of 1789 and the Terror that followed in 1793 brought most academic research to a halt. At first Laplace kept his head down, contributing to the debates of the time only on their strictly scientific aspects, as, for example, in the definition and promotion of the metric system. In 1796 he published his first contribution to the public understanding of science, his *Exposition du système du monde*,8 a taster for the treatise that started to appear two years later. Even before Napoleon took control, things had stabilized and the First Class of the Institut had replaced the defunct Académie des Sciences. Laplace was drawn more and more into public life, even serving as Minister of the Interior for six weeks. He later became a Senator, and, after the restoration of the monarchy in 1815, a Peer. His concern with public affairs was one of the sources of his interest in the theory of probability, another being the use he made of it in his astronomical calculations. Hahn shows how these concerns led him to the inverse problem of probability—how to deduce causes from events, a field in which he was initially unaware of the earlier work of Thomas Bayes.

For some years after 1806 his scientific work centred on the Société d'Arcueil, a private laboratory and discussion group that he and Berthollet set up at their neighbouring properties in the village of Arcueil, just south of Paris. Again, Hahn gives a sound account of the work of this group on what we should now call classical physics, but without going into detail. He places some emphasis on Laplace's unwillingness to entertain ingenious notions that lacked experimental evidence. Eventually, however, it was some of Laplace's firm but unfounded convictions that were to undermine his contributions to physics when a new generation, although reared on his mathematical methods, turned from such ideas as a corpuscular theory of light and a caloric fluid responsible for the repulsion between molecular particles. Moreover he, like all his contemporaries, held to a static picture of matter; ‘kinetic theory’ gained ground only slowly in the first half of the nineteenth century and but in the second half it became an essential part of molecular physics.

Every page of Hahn's clearly written book provides evidence of his mastery of the period and of the surviving manuscript material. His treatment of Laplace's science is always to the point, but without the technical detail, and his assessment of Laplace's position on matters of philosophy and religion can surely be relied upon. This work must become the definitive biography of Laplace, the man, but for an account of how he accomplished his mathematical and scientific work we must still turn to Gillispie's book.

## Notes

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P. S. Laplace,

*Oeuvres*, 7 volumes (Imprimerie Royale, Paris, 1843–47);*Oeuvres complètes*, 14 volumes (Gauthier-Villars, Paris, 1878–1912). - ↵
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C. C. Gillispie with R. Fox and I. Grattan-Guinness, ‘Laplace, Pierre-Simon, Marquis de’, in

*Dictionary of scientific biography*(ed C. C. Gillispie), vol. 15 (Scribners, New York, 1981), pp. 273–403. - ↵
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The Royal Society copy of this book is a gift from Laplace. He, Berthollet and Legendre had all been elected Fellows a few weeks before the storming of the Bastille in 1789.