Madame Curie, the discoverer of radium and of polonium, was a woman of passion. The popular view of her life is a tale of almost elemental force of character.

Having arrived in France as a penniless Polish student, she went on to dominate French science at the beginning of this century. She was awarded two Nobel prizes, discovered two chemical elements, essentially invented the concept of radioactivity and founded a scientific dynasty: her daughter Irene and son-in-law Frederic Joliot-Curie themselves shared a Nobel prize in the Thirties. (Marie was the first - and Irene only the second - woman to win the Nobel prize.)

Yet Madame Curie's passions were not confined to her professional and scientific life. In 1910, about four years after her husband, Pierre, had died in a road accident, the 43-year-old widow embarked on a highly charged love affair with Paul Langevin, a scientist five years her junior. The lovers even rented a flat near the Sorbonne where they could meet in secret.

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There was a drawback. Langevin was a married man and the father of four children. Langevin's wife discovered the love letters that Marie had written to him and dished the dirt to the Parisian equivalent of the News of the World. According to Susan Quinn's recently published biography Marie Curie: A Life, rumours of an affair had already been circulating.

Publication of the letters scandalised France. It was clearly not just a physical infatuation. Marie was thinking in terms of marriage and had written to her lover urging him to divorce his wife and marry her, although that would scarcely have been any less shocking at the time. Moreover, Paul Langevin had clearly not completely given up on his own marriage: his wife bore their fourth child just before he embarked on the affair with Marie.

After the news broke, the Swedish Academy of Sciences tried to dissuade her from coming to Stockholm to receive her Nobel prize so that the adulteress should not shake hands with the Swedish king. Paul Langevin felt honour- bound to fight a duel against the journalist who wrote the expose. He arranged a legal separation from his wife, but despite Marie's urgings, refused to seek a divorce. Her reputation was not completely restored until her heroic efforts to help wounded French soldiers during the First World War.

Had Marie Curie been an artist this scandal might have been unremarkable. It has long been accepted that an understanding of an artist's life illuminates and enhances one's understanding of their work. That Joe Orton, the Sixties playwright, lived in a sort of homosexual marriage with Kenneth Halliwell (who murdered him) and that he went "cottaging" in public lavatories is relevant to the black humour of Orton's plays. Is the same thing true in science?

Genius, James Glieck's recent masterly biography of Richard Feynman, revealed the world's greatest post-war theoretical physicist as a notorious philanderer. He slept with many of his colleagues' wives, was a regular visitor to strip clubs, and on one occasion appears to have financed his mistress's illegal abortion (although the details are inevitably murky). Even Einstein, that icon of the ethereal scientist, had an enthusiastically earthy side when it came to women. Is there a connection between the energy and drive these people brought to their scientific practice and the power of their emotional lives?

Marie Curie was in no doubt about it. When Svante Arrhenius, a member of the Swedish Academy of Sciences, wrote to her after the story of her love affair broke, she responded briskly: "The prize has been awarded for the discovery of radium and polonium. I believe that there is no connection between my scientific work and the facts of private life. I cannot accept ... that the appreciation of the value of scientific work should be influenced by libel and slander concerning private life."

Personal biography is important to understanding science because there are different personal styles of scientific research. In the best book yet written about modern biology (recently reissued in paperback), The Eighth Day of Creation, Horace Freeland Judson remarks that "as scientists understand very well, personality has always been an inseparable part of their styles of inquiry, a potent if unacknowledged factor in their results. No art or popular entertainment is so carefully built as is science upon the individual talents, preferences and habits of its leaders." Judson contrasts the thorough, intense style of Max Perutz, who won the Nobel prize for discovering the structure of haemoglobin, with that of Jim Watson, the co-discoverer of the double helix of DNA.

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Perutz began working on haemoglobin in 1937, but even though he worked with great dedication on the problem, he did not achieve full results until 1958 - 21 years after he started. Watson, on the other hand, has never worked on any problem for more than a couple of years. Yet the realisation that DNA had the structure of a double helix is probably the single most important scientific discovery of the second half of the 20th century. Watson's famous account of his partnership with Francis Crick, The Double Helix, conveys the impression that Watson spent most of his time playing tennis and chasing pretty girls. He still plays tennis, and even today there are traces of that impatience to get on with things: Watson never finishes a sentence, always swallowing the last few words as he moves on to the next thought.

There is one common factor in the biographies of successful scientists: to a surprising extent, scientists are social outsiders. The penniless Polish girl Maria Sklodowska was an outsider simply by virtue of her sex: she had to go to the Sorbonne because Warsaw University did not admit women. Marie was a foreigner in France and from not quite the right social class in Poland. Her husband, too, was for most of his life a non-establishment figure. When they began their research, it was in a laboratory at the Ecole municipale de physique et chemie - an establishment definitely lacking in social cachet. It is difficult to believe that this aspect of their biographies is not relevant to understanding how they came to do such creative scientific work.

The tension between social class and science is particularly evident in Britain, especially from the beginning of the 19th century. The impact of the dark satanic mills of the industrial revolution, combined with the respect for the classics engendered as part of Thomas Arnold's reforms of the English public schools, helped to spread the Platonic view that experimental science was not the occupation of gentlemen. Science has always been a craft-based activity and the best scientists tend quite literally to be "hands on". (It is no accident that the key to Watson and Crick's discovery of the double helix was that they made physical cut-out models of the molecule's component parts and tried endlessly to fit them together.)

Gentlemen do not get their hands dirty in that way. So the greatest of British scientists have tended to be from the fringes of the UK, geographically or socially. The two towering figures of 19th-century British science were Michael Faraday and James Clerk Maxwell. Faraday, Mrs Thatcher's favourite scientist, was the son of a blacksmith and an adherent to an obscure religious sect, the Sandemanians. Maxwell, the brilliant theoretical physicist, was a Scot.

In the early 20th century, Marie Curie's contemporary, Ernest (later Lord) Rutherford was Britain's greatest experimental physicist: he came from New Zealand. Watson and Perutz were, of course, foreigners to Britain and while Francis Crick comes from comfortable middle-class stock, his scientific biography is unusual in that, by the time he started working with Watson, he, too, was a scientific outsider: he was over 30, had essentially failed as a physicist, and had still not obtained his PhD.

Science is not done by cold, logically correct, calculating machines. It is an intensely human activity, just as the arts are. Science requires inspiration and imagination, which means that personality, passion, and style are important. And while Marie Curie seems to be right that scientists' sex lives do not matter, it appears that their social class does.