ART / The bigger, the worse: For its new exhibition, the National Gallery has taken a short cut and imported the contents of a provincial French museum. Andrew Graham-Dixon looks round and wonders if he might be on holiday

The National Gallery's new exhibition, 'Tradition and Revolution in French Art 1700-1880', might look like a case of thinly disguised curatorial opportunism. The show owes its existence to the temporary closure of the Musee des Beaux-Arts in Lille, whose director had the bright idea of loaning the bulk of his collection of French art to Trafalgar Square while he had the builders and decorators in. The National Gallery, not especially noted for the strength of its collections of French 18th- and 19th-century art, was understandably receptive to the proposal. But whether the contents of a single provincial museum can be said to give a coherent account of the vexed relationship between academic conformism and innovation in French art during a period of nearly 200 years is - notwithstanding this exhibition's impressive title - somewhat debatable.

No museum's collection can be tidied up into anything as logical and lucid as the exposition of an art historian's thesis. But that is not a bad thing, and one of the pleasures of this show is the unevenness of its contents. Unquestioned masterpieces hang beside works of hopeless but fascinating mediocrity. Great but also minor painters wrestle with their own private demons. The exhibition has the lively incoherence, the enjoyable inconsistency of - well, one of those French provincial museums that you might visit while on holiday, during a spare afternoon, more out of curiosity than with any great sense of expectation. And, however unpredictably, it also manages to tell a sort of story (full of hesitations, false starts, intricate subplots) about the development of French art.

One of the incidental lessons of this show is that grand subject matter is no guarantee of artistic achievement. One of the more ambitious paintings in the exhibition, Jules Bastien-Lepage's Achilles and Priam, is perhaps its most unmitigated pictorial disaster. Bastien-Lepage took his subject from the Iliad, Book 24 - old King Priam humbles himself before Achilles in order to recover the body of his slain son, Hector - and said of his picture that 'I want it to be heartbreaking to see this noble and wise old man kneeling down in front of the leader of the savages who coveted the treasure of Troy'. He produced an extraordinary conflation of crazed sentimentality and fey eroticism: the wizened, kneeling Priam snivelling all over the flatteringly spotlit body of Achilles, naked except for the painter's idea of what an ancient Greek G-string might have looked like. Bastien-Lepage's high moral ambitions issued in what is liable, these days, to look like high theatrical camp - King Lear meets the Chippendales.

Moving from the ridiculous to the sublime (as this show often invites you to do), you encounter Chardin's unassuming, unambitious and utterly remarkable painting, The Silver Goblet. The picture's title is misleading: Chardin has actually painted a silver goblet, a corked bottle of wine, a pewter plate and a loaf of bread into which a bone-handled knife has been stuck, all standing on a shadowed stone ledge. Ordinary things, extraordinarily painted; the aftermath of a meal remade as a form of pastoral, a still life that seeks to represent, in microcosm, the virtues of the simple life. Chardin's great gift was his ability to make the mundane seem consequential, to aggrandise (but inconspicuously, subtly, without banging a drum) everyday activities like eating, cleaning, unpacking the groceries. The painting may even covertly refer to its own transformative potency, its transubstantiation of ordinary stuff into an image fit for moralised contemplation: the bread and the wine here suggest a parallel between the miracle of the Mass and the miracle of art. Chardin may have been less modest than his reputation suggests. (He was right to be.)

From Chardin in the early 18th century to Bastien-Lepage in the late 19th, this exhibition demonstrates the peculiar fluidity of French art. It makes visible, in its own, necessarily haphazard way, a fruitful confusion about the meanings and uses of the traditional genres (conventionally ranked in descending order: history painting, portrait, landscape, still life) that goes to the heart of French art of this time, and after. This confusion would eventually lead to the gradual collapse of the old academic distinctions between high and low subject matter - and then on to the modernist free-for-all where the grandest ambitions of the artist might express themselves in a picture of nothing more apparently conseqential than, say, an apple. Chardin and Bastien-Lepage show, between them, how grand the lowest subjects can be made to appear and how irredeemably trivial the highest can seem, how debased by rote. But the entire show is full of such unexpected role reversals, full of paintings that, intentionally or otherwise, defeat the expectations created by their subject matter.

This is certainly true of one of the most remarkable paintings in the show, Courbet's Une Apres-dinee at Ornans. What it represents, in the context of this exhibition's story of French art, is one form of rule-breaking. Courbet's painting might be described as an example of confrontational pastoral, pastoral aggrandised far more brusquely and provocatively than ever would have been deemed possible in the days of Chardin. It is, effectively, only a genre scene: four men, the painter among them, seated around a dinner table in a rustic cottage, united in a mood of post-prandial reverie. But the painting's combativeness inheres in its scale: it is enormous, a picture of nothing much, painted on the scale of a history painting. It was Courbet's aggressive way of announcing the imminent death of the grandiose tradition of academic narrative art. Courbet's picture was a rebuke to the Salon's dreamily academic world of pneumatic gods and goddesses - represented in this show by Amaury Duval's The Birth of Venus, a coy porcelain nude that sees the classical tradition of figure painting terminally debased, remade as source material for the Athena posters of the future - and it was the manifesto of an emerging realist movement in France. (Although Courbet's realism, like all realisms, is actually a fantasy: Une Apres-Dinee at Ornans is an idealised vision of simple country existence, rural life seen through a veil of sentiment.)

The two greatest pictures in this exhibition might be said to demonstrate curious metamorphoses of the sort of grand narrative painting that Courbet would so vigorously reject. The first, David's Belisarius, was painted in 1781 and was seen at the time to represent a brave new dawn in French painting. Diderot saw David's first masterpiece as the way forward for French art. He had spent at least a decade calling for an end to rococo frivolity ('Hasn't the brush been too much and for too long devoted to debauchery and vice?') and David's tragic image of the worthy-but- wronged Roman general, blind and impoverished, touching the hearts of a noblewoman and her child, seemed like the answer to his prayers. But David's painting is hardly a history painting in the conventional sense: it draws its unusual power, its tearjerking effectiveness, not from the grand tradition of Poussin but from a very different form of painting, the sort of affecting domestic genre pictures painted by Greuze. David's picture is Greuze in antique fancy dress: history painting for the age of sensibility; history painting to make men of feeling cry.

Forty years later, Theodore Gericault in turn redefines history painting in his extraordinary oil sketch for The Race of the Riderless Horses, an enormous picture (now lost) which he planned for nearly a year but never completed. Gericault paints narrative painting minus the narrative: an image of men and horses struggling in Stygian gloom, a paroxysmic version of the Parthenon frieze which seems to predict the violent, furious energies of Picasso's subject painting. Here, and in Delacroix's extraordinary explosion of a still life painting, his Bouquet Champetre - a picture of flowers which turns out, surprisingly, to be far more passionate and truly Romantic than his Medea, a picture of a mother murdering her children under darkening skies - you sense modern art about to happen.

(Photographs omitted)