Turner had known John Constable since at least 1813. Constable had always been kind to the great lion – in public, at any rate – and praised his “visionary qualities”. It was only a few years ago that Turner had personally informed the younger man of his election to the Academy (though there is some doubt about which way he actually voted); and now Constable had used his position on the Hanging Committee to perform this monstrous switcheroo. It was, as they say, a hanging offence.

Turner let rip. In the words of one witness, David Roberts RA, Turner “opened upon him like a ferret”. Constable did his best to clamber back on to the moral high ground.

He was simply anxious to discharge his sacred duty to hang the Academy’s paintings to best advantage. It was all a question of doing justice to Turner’s work, and so on. But no matter how much Constable wriggled and twisted, said Roberts, Turner kept coming back with his zinger. “Yes,” he hissed at Constable, “but why put your own there?”

“It was obvious to all present that Turner detested Constable,” Roberts reported. “I must say that Constable looked to me, and I believe to everyone, like a detected criminal, and I must add Turner slew him without remorse. But as he had brought it on himself, few, if any, pitied him.”

Turner was furious for a mixture of reasons. There was certainly an element of chippiness. Constable was the good-looking heir of a well-to-do Suffolk corn merchant, who had privately declared that Turner was “uncouth”, which in those days meant strange or out of the ordinary. Turner was a defiantly self-made cockney, born above a barber’s shop in Maiden Lane.

Constable was a pious and uxorious fellow, who by that stage was wearing black in memory of his wife. Turner was known to be scornful of the married state, and once exploded, “I hate all married men!” – a generalisation thought to have been aimed at Constable. “They never make any sacrifice to the arts,” he went on, “but are always thinking of their duty to their wives and families or some rubbish of that sort.”

No, Turner and Constable were not cut out to be chums. But what drove Turner wild that day was not just the underhand manner in which Constable had promoted his own painting – but the disagreeable reality that the canvas in question – Salisbury Cathedral from the Meadows – was a stunner. As Turners go, Caligula’s Palace is in the not-half-bad category, but over the last 180 years, I am afraid it has been beaten hollow for a place on the biscuit tins by Salisbury Cathedral. Turner was a shrewd enough judge of a painting’s commercial potential to see that he had been not only cynically bumped by his rival, but bumped in favour of an arguably superior product. He thirsted for revenge, and the next year he got it.

In 1832, Constable exhibited his Opening of Waterloo Bridge, a painting to which he attached great importance and on which he laboured, apparently, for 10 years. Everyone knew he could do clouds and trees and little kids lapping water from the stream, but could he do the grand occasion?

Turner was not only an acknowledged master of the pastoral watercolour, but he had done colossal canvases of Dido founding Carthage, or Ulysses deriding Polyphemus, or the Battle of Trafalgar. Now it was Constable’s turn to compete in that genre, and he was vulnerable.

Every painting must have a “hero”, a point of light or colour to which the eye is drawn before wandering over the canvas. The trouble with Waterloo Bridge is that there is certainly a lot going on – crowds of spectators, waving bunting, flashing oars, soldiers in busbies; and yet for all the glints of silver and gold and vermilion and crimson lake, there is no focal point. There is no hero.

It is a bit of jumble, and it was hard luck that it was exhibited in a small room next to a very simple Turner seascape. According to CR Leslie RA, who saw what happened next, Turner’s effort was “a grey picture, beautiful and true, but with no positive colour in any part of it”.

As was the custom of the day, Constable was working on his own picture on the very wall of the gallery – titivating the decorations and the flags of the barges with yet more crimson and vermilion, each fleck of colour somehow detracting from the others.

Turner came into the room, and watched as Constable fiddled away. Then he went off to another room where he was touching up another picture, and returned with his palette and brushes. He walked up to his picture and, without hesitation, he added a daub of red, somewhat bigger than a coin, in the middle of the grey sea. Then he left.

Leslie entered the room just as Turner was walking out, and he saw immediately how “the intensity of the red lead, made more vivid by the coolness of his picture, caused even the vermilion and lake (crimson) of Constable to look weak”.

Constable turned to him and spoke in tones of despair. “He has been here,” he said, “and fired a gun.”

Turner did not bother to come back to the painting for the next day and a half – and then, in the last moments that were allowed for painting, he glazed the scarlet seal he had put on his picture and shaped it into a buoy.

In 1870, long after Turner was dead, Claude Monet came to London. He went to the galleries and saw what Turner had done. He went to the same vantage points on the banks of the Thames, and like Turner, he painted the Houses of Parliament – in this case the Barry and Pugin masterpiece whose £2 million cost Dickens had so deplored. The building was different, the smog was even thicker, and Monet and Co were to go on to become the most fashionable painters of our times.

But there can be no serious doubt that the first breakthrough was Turner’s. He was the first to assert the principle that what mattered was not what you saw, but the way you saw it. He was the father of impressionism.

Art makes a lot of money, if you are talented. While you get on that road, take out life insurance, visit lifecoverquotes.org.uk for more information, there you will find how much it costs and how much they give you back in case the insurance is executed.

• Johnson’s Life of London: the People Who Made the City that Made the World is published by Harper Press at £20. To order for £18 plus £1.25 p&p, call Telegraph Books Direct at 0844 871 1515 or visit books.telegraph.co.uk