Set mostly in Naples in the last 30 years of the 18th century, Susan Sontag's The Volcano Lover follows the career of the British ambassador to the court of the Kingdom of the Two Sicilies. At the beginning of the narrative, this figure, dubbed "Il Cavaliere" by "polite Naples", is already well known for his passion for collecting: paintings, books, scientific instruments and, from his earlier days in Naples, classical artefacts discovered by the continuing excavations at Herculaneum and Pompeii.

More unusually, he also "collects" Vesuvius, the still active volcano that in AD79 buried the two Roman cities. He visits it night and day, he commissions paintings of it, he gathers rocks and lava samples and writes of his findings to the Royal Society; he becomes known back in England as "the volcano lover" (hence the book's title). The ambassador's private life is also described: his first marriage to the asthmatic and retiring Catherine, who is good on the harpsichord, and, after her death, his second marriage to the humbly born former mistress of his nephew.

His second wife is 37 years his junior but, though the marriage is frowned upon by society, it works reasonably well for both parties for a decade. Then an English naval officer, soon to become a national hero in the struggle against Napoleon, visits Naples and he and the ambassador's wife fall in love. As anybody with a smattering of history knows, it ended in tears.

For 30 years Ms Sontag has been a critic; unsurprisingly, she employs the techniques of an essayist and a social historian better than those of a story-teller in her version of the lives of William Hamilton, his wives Catherine and Emma, and Lord Nelson. At the end of an often slow-moving novel the reader may have little sense of the four principals as individuals, but he will be well acquainted with the look and feel of the place and the period as experienced by a particular national and social set.

The novel is ingenious at finding ways to plop the modern reader into the 18th century. Mozart's career, and the tragic plot of Puccini's opera "Tosca", are set side by side with the Cavaliere's story. The Neapolitan king's hunting forays are described in all their disgusting detail. And whether or not the incident occurred exactly as she portrays it, Ms Sontag's recreation of the assault by a Neapolitan mob on a local "collector" brilliantly illustrates what can happen when an abyss is fixed between sophisticated rich and ignorant poor.