Yet it was never finished. Volumes One to Three appeared quickly within the space of a few years, covering Rome's history during the so-called "Republic" - a period of constitutional government under the Senate and elected magistrates - that Mommsen enthusiastically saw as a model for the new nation-states of Europe; Volume Five followed 30 years, later (a much drier history of the various provinces of the Roman empire). Volume Four, which was to cover Rome under the emperors, after the tyranny and assassination of Julius Caesar, never appeared. There was no doubt what Mommsen thought of this period; he repeatedly referred to it in his writing as a "deeply degenerate age" of "leaden tedium" destroyed by "inner putresence".
In 1980, however, a young German historian walked into a second-hand bookshop in Nuremberg, and came across a pile of old notebooks. These turned out to contain students' notes from Mommsen's courses on the history of the Roman empire in the 1880s - the closest thing, as their discoverer instantly realised, that we would ever get to the missing Volume Four. Edited into connected prose, they were published in Germany in 1992 to front-page headlines and they are now translated into English.
The serendipity of this whole story is astonishing; the chances that (even in well-educated Nuremberg) the books would have been spotted by someone who actually recognised what they were are almost too small to contemplate. But the more hard-headed response must ask whether the final published version lives up to its promise.
There are indeed some engagingly iconoclastic soundbites buried here. Mommsen was a notoriously outspoken oddball - in his academic life as much as in his modern politics (his remarks on Gladstone's Irish legislation, for example, were said to be unprintable). By the 1880s he already had an established reputation as the greatest defender of Roman history and culture in a Germany that was still enthralled by Hellenism. His audience was presumably meant to be amused, as well as shocked, when they heard him dismissing Rome's national poet Virgil as a "sorry comparison" with the Greek Homer, chastising the central episode of his Aeneid, as "a vulgar erotic motif" and lamenting that the whole poem was not burnt on Virgil's death. But such passages are, within a rather dreary historical narrative, few and far between.
The editors do a valiant job in their introduction, trying to catch something of the spirit of this extra-ordinary man: a parodic workaholic, who was once found at seven in the morning outside the Bodleian in Oxford, complaining that the library was not open till nine; a committed citizen, who was for years a member of the Prussian Parliament; a prolific Roman historian, who singlehandedly rediscovered (though some would now say, more critically, "invented") the whole legal basis of the ancient Roman constitution. All the same, to re-read these lectures on the history of the Roman empire inevitably prompts the suspicion that, for all his promses, his failure to produce Volume Four was a calculated (and wise) decision. For once, perhaps, he didn't have much he wanted to say.