The grande halle built in the style of the Eiffel Tower, plus the satanically- secret society who worked in it, held irresistible charm for the well- heeled of Thirties society.
When the slaughterers decamped, the politicians had a dream worthy of elephant-king Babar in its naive utopianism: they would create a pleasure park, with a Cite des Sciences at one end, and a Cite de la Musique at the other.
Ten years on, that dream has indeed become a utopia. The park is laced with waterways, and punctuated by bright red follies by the futurist architect Bernard Tschumi. The Cite des Sciences is packed with state-of-the-art interactive gadgetry, while its planetarium, housed in a polished-steel bubble, looks ready to slip its moorings and sail into the empyrean.
There's hardly a straight line to be found in the Cite de la Musique, which looks as if it's been sculpted from cardboard. The windows are punched-through like portholes; the furniture is Philippe Starck on tranquillisers. Visual musical gags waylay you at every turn. Each half-hour, 100 metronomes in a black box are set off simultaneously at different speeds (this is a "work" by Ligeti). You might be in a laboratory, or on the set for Covent Garden's enfant-terrible Ring. It's so antiseptically clean you might be in a hospital. Then you learn that Christian de Portzamparc - who won the competition to create this place - had to measure up to a singular challenge: "The designer is invited to compose a piece of music." A symphony in walls and windows, then, unsullied by practicality.
As the Cite's director Brigitte Marger explains, there were two other presiding geniuses at its birth: culture minister Jack Lang, and the composer/conductor Pierre Boulez. That birth might easily have been an abortion, thanks to Mitterrand's sudden decision to divert Cite funds to build the Bastille Opera. "The spirit remains," says Marger, "but the project was severely diminished. We lost our symphony hall."
No matter. The existing auditorium is a remarkably flexible space which the South Bank wants to replicate, and it's drawing a better crowd than Marger ever dreamed of. The performers are top-flight international, while the audience are predominantly young and local. With the Conservatoire in residence, and the Musee de la Musique open at last - with its ancient and modern riches, and its period- instrument recitals - the original vision is almost complete.
There are stresses and strains in the best-regulated families: the fight here turns on what museums are for. Trouble started when the staff took Marger to court and won a ruling which exempted them from incorporation into her Cite framework. Marger's response was brutally simple: if they were outside her jurisdiction, she could not pay their salaries. Oops! On appeal, her jurisdiction was reaffirmed. Moreover, there's strife between the museum and the Conservatoire over which of the museum's instruments should be played. The Conservatoire think regular playing keeps string instruments alive; the museum people say it damages them. Marger has just set up a committee with representatives from each side to adjudicate each case.
Museum director Marie-France Calas says sweetly that "a little friendly tension" is normal, but she goes on to talk with a shudder of the way successive generations of students were allowed to destroy their instrumental heritage. "I remember a lute-player modifying a period lute for a radio broadcast. That was a crime." There are two warring belief-systems, she observes: la logique du spectacle, et la logique du patrimoine. When money permits, she commissions the building of replicas - "and those are the instruments that should be played". That initial impression of mine is borne out by a visit to the workshop and research archive: the place is indeed a laboratory.
It is also a mission station, sending the gospel via teachers to every school and via students to the ends of the earth. The French foreign ministry spends one third of its budget on culture. Marc-Olivier Dupin, the Conservatoire's new director, has brought commonsense to bear on some traditionally vexed questions: students may still compete for their prix, but they can only graduate if they also get the new diploma in all-round musicianship. "In the German Hochschulen," says Dupin, "there are 600 flautists at any given time. How many of those will get jobs?"
Tuition charges at the government-funded Conservatoire are nominal. This means that, unlike its British and American counterparts, it is relatively open to foreign students. The end-of-year performances I hear include recitals by a Turkish baritone and a tenor from Francophone Africa, and I meet Morocco's first-ever counter-tenor, still smarting from his rejection by London's Guildhall. Since Rashid Benabdesalem now sings with Les Arts Florissants, the Guildhall will doubtless hear him soon, whether they like it or not. Age is no barrier at the Conservatoire: I meet Jean Dube, a 15-year-old pianist who enrolled when he was 11.
I also meet his mother, who has monitored his every move since he left school at eight, and who - to the dismay of his professors - acts as his manager. He already has a career, and is preparing for his next Japanese onslaught by learning the language. Power-packed and almost spherical, he whirls through some Liszt while she looks on with approval.
Performers are the same the world over. It's the creators and theorists who take colour from the soil. Young musicologists here are still falling over themselves to study Schoenberg. The Baroque fad is fading, Birtwistle is in the ascendant and Xenakis in a decline. The American minimalists are championed by conservatives.
Much of their work could have held their own in this year's Proms. It's clear that Boulez - flanked by his Ensemble InterContemporain - is still the presiding genius, but there's much originality here.
Unlike their dour mentor, these young composers are setting out to beguile the ear. The big question is what happens when the old boy moves on: will the garden go to seed?