A taste of hell

Delacroix was more than a mere romantic painter, says Jonathan Jones. He was the source of all modern art's depravity
Detail from Delacroix's The Death of Sardanapalus
Frightful abandon: detail from Delacroix's The Death of Sardanapalus

Modern art began in France. That much is indisputable. But when, and with whom? Many founding images and artists are plausible; you could begin the history of modernism with Manet's Olympia (1863), or Courbet's Burial at Ornans (1849-50), or even David's Marat Assassinated (1793). But 19th-century Parisians would not have agreed with any of these claims.

For them, the founder of the new art was unquestionably Eugène Delacroix. "The majority of the public," wrote Charles Baudelaire, poet, provoker of public morals and art critic, "have long since, indeed from his very first work, dubbed him leader of the modern school."

Baudelaire worshipped the painter as a dark god: "Delacroix, lake of blood, haunted by evil angels," he called him. Delacroix didn't agree with everything Baudelaire wrote about him. He was baffled when the poet, who also translated Edgar Allan Poe's horror stories, compared his favourite painter to his favourite writer; after all, Delacroix was a successful, celebrated and highly self-disciplined artist, showered with state commissions and in his later years painting severe religious works in St Sulpice, whereas Poe was an alcoholic who married his child cousin and was found dying in the streets of Baltimore. Delacroix was not a bohemian. But his imagination was the source of all modern art's depravity.

Delacroix was different in nature from any other painter of the early 19th century. He was acclaimed as the leading French Romantic painter for the masterpieces he painted in the 1820s, when he was still in his 20s - The Bark of Dante and Virgil (1822), The Massacre at Scio (1824), The Death of Sardanapalus (1827). But these paintings have as little in common with other Romantic art as Poe's tales have with Walter Scott's - in place of the humanism, historical seriousness, novelistic detail and naturalism of other early 19th-century painters, Delacroix poisons the mind with a sinister, corrupt, and at the same time physically lighter, more elusive romanticism, with the same narcotic appeal as Poe's tales and Baudelaire's verse; a romantic art that we recognise as modern in its morbidity, cynicism and abstraction.

You have only to compare Delacroix to the British painters he admired to see how different he is. Whatever it was that he learned from Constable's Haywain, which he praised effusively at the Paris salon in 1824, it wasn't anything central to his own art. Constable and Turner are in a different world from Delacroix: moral, sublimated, their art powerful precisely because of its introspective distance from modern urban life. Delacroix is introspective, poetic, but he is not a moralist. He freed painting from the pieties of naturalism - nature, for him, said Baudelaire, was a "dictionary", which he consulted in order to use its language to create works of free imagination.

The difference between Delacroix and his British contemporaries is summed up by the contrasting critical voices of Baudelaire - hedonist, abstractionist - and John Ruskin, Turner's champion, a man for whom art must be moral above all else. Ruskin was to denounce Whistler, a modernist, as an amoral "coxcomb" throwing paint in the public's face.

The exhibition Constable to Delacroix, opening soon at Tate Britain, argues for the influence of British artists on the French - but this is a half-truth that smacks of nationalist wishful thinking. What Delacroix took, he translated into something unrecognisable. Even the entry in his journal praising Constable suggests how alien he was to 19th-century British culture: after saying that seeing Constable's Haywain at the Louvre "did me a world of good", he records his extreme sexual frustration - not the kind of frankness we associate with Constable.

What was missing from British art in the 19th century was any sense of bohemia, of the avant-garde, of artists rejecting society. Like other Victorians, 19th-century British painters were terrified of the gutter, the poorhouse, the void of the city at night. French artists saw the beauty as well as horror of that void: it was Delacroix who taught them to.

Emptiness appears in French art of the 19th century in a way it had never been allowed to in previous painting - not, this time, the emptiness of the unfinished, which had been savoured in 18th-century art as an index of sensibility, but something more disturbing; an intimation of nothingness at the heart of carefully, even academically constructed images.

It is not quite true to say that Delacroix had no predecessors. He had one, Géricault, who encouraged him, and used him as a model for one of the figures in The Raft of the Medusa. This massive painting centres on a hole, an absence: there is a black gulf at the heart of the scene of desperation.

But it is Delacroix who makes insubstantiality and inconstancy of space his organising principle, so that emptiness is not just figured in his paintings but seems to be what they are made of, or in. The Death of Sardanapalus is his most radical parody of history painting. It hangs in the Louvre, in a gallery that displays the great public paintings done for the Paris salon in the 18th and early 19th centuries. So you can see how the earnestness of David's Oath of the Horatii or his giant canvas of Napoleon crowning Josephine is mocked by the decadent painted world of The Death of Sardanapalus, in which every detail seems to revel in its fatuity; in which colour is the only reality and bodies are obviously insubstantial, transparent fantasies.

This is an empty canvas on which the mind projects its daydreams - and what a daydream. Sardanapalus floats on a vast pink divan with gold elephant-head decorations; around him cascade bodies and treasure, as his court is massacred to accompany him in his planned suicide. The bodies, human and animal, and the jewels and gold, and that great bed - a sleazy version of Géricault's Raft - seem not to obey any laws of gravity. In fact, there is no real, oxygenated, mappable space in the painting: it is a billowing tapestry, a series of planes of colour tumbling over one another. It is an abstract painting.

Delacroix is a great colourist - that is a truism; what matters is what he does with colour. His paintings do not use colour to evoke the natural world, but to trigger far stranger, more inward associations, and to suggest bodies, emotions, relationships, without ever pinning anything down.

Like Poe's tales, his paintings generate atmosphere, and the physical worlds they depict are unreliable - space vanishes, replaced by fantasies of colour. In his Women of Algiers, it is difficult even to reconstruct the space depicted; the women float in a sober yet luxurious other world whose red door might be just a painting of a door.

It would be wrong to see Delacroix just as a sensationalist. He was a poet - he wanted to replicate the effects of poetry in painting - who started his career with a painting of Dante visiting hell. The Death of Sardanapalus is inspired by a verse play by his favourite modern poet, Byron, the one British figure who really was a crucial influence on 19th-century French art. Byron (always called by them Lord Byron) exemplified the figure that Delacroix and succeeding Parisian generations identified with - the dandy, held to be a particularly British character, epitomised by Lord B and Beau Brummel. Delacroix said that in his youth, one of his main ambitions was to introduce British fashions to France; his portrait of Louis Auguste Schwiter (1826) in the National Gallery, which he completed after a visit to Britain, is a consummate image of the thin, dark-suited dandy in the English manner.

This painting is a poignant example of Delacroix's colourist genius; next to the man in black, a shiny blue vase glitters, a strange, feminising juxtaposition. Where Turner is all white, yellow and fiery manhood, Delacroix is a connoisseur of more degenerate, dandified colours: greens, pinks, reds, blues, a palette that smells of rain on city gardens.

It was the subtlety, rather than the wildness, of Delacroix that had the most profound influence on French art. He inspired poetry as he had been inspired by it; Baudelaire's writing on Delacroix is central to his aesthetics, and his understanding of Delacroix pervades his own verse; when he pictures his lover "dans l'enfer de ton lit" (in the hell of your bed), you cannot help but think of Sardanapalus.

The unexpected, savage, yet understated colours of Delacroix, and the abstract worlds that his palette created, perfume French art; Delacroix is there in Olympia's flowers, Gauguin's horses, Matisse's studio. He was there with Van Gogh at the night cafe. What he taught French modern artists was later put into words by Gauguin as a symbolist manifesto: "Soyez mystérieuses."

· Constable to Delacroix: British Art and the French Romantics is at Tate Britain, London SW1 from February 5. Ticketmaster: 0870 166 8283.