The Duke himself had the taste and enthusiasm to form a superb collection - an admirable act in itself. His nephew and heir, the Marquis of Stafford, later 1st Duke of Sutherland, built a gallery to house the pictures which he opened to the public in 1806 - a public-spirited move far ahead of its time. It pre-dated the foundation of the National Gallery by almost 20 years.
The paintings have been passed down through the family, and the best of them have been on public exhibition ever since. The present Duke of Sutherland has 27 pictures, worth some £250m, on loan to the National Gallery of Scotland. Rather than selling the paintings for their own gain, the family are allowing the public to enjoy them for nothing.
In no other country have aristocratic art collections survived the way they have in Britain. Carefully honed tax incentives, introduced by successive governments, mean that most art here is on public view, directly enriching our national life. The present government, however, is said to be considering whether to abolish the acceptance of works of art in lieu of tax, a system which helps keep great art in this country. The story of the Bridgewater collection demonstrates better than any other what is at risk if the owners of historic art collections are alienated by tax changes of that kind.
The collection now belongs to the 6th Duke of Sutherland, a very private country gentleman who lives in the Borders and will be 80 next month. His heir, Francis Egerton - a 54-year-old Suffolk farmer - tells me he would like the pictures to stay at the National Gallery of Scot-land, but can't commit himself because of "the awful uncertainty of taxes and things like that".
There is no other "national gallery" where the best pictures on view still belong to a private owner. The National Gallery of Scotland has had three Raphaels, five Titians, three and a half Rembrandts (the "half" is a fourth painting now deemed to be by a Rembrandt follower), eight Poussins, a Tintoretto, a Van Dyck and a Hobbema on loan from the Duke since 1946. A Rubens sketch, borrowed for a special exhibition last year, has now been added to the long-term loans.
The loan of the best paintings to the National Gallery of Scotland was negotiated in 1945. Up until 1939, they had been on public show in a gallery at Bridgewater House - the family home in London - but it was decided to remove them to Scotland to get them out of Hitler's reach. The Germans duly bombed the London gallery and it never reopened.
The real story begins with the 3rd Duke of Bridgewater, who was born in 1736 and inherited the title at the age of 12. At 22 he was briefly engaged to one of the lovely Gunning sisters, two renowned Irish beauties, but broke off the engagement and devoted the rest of his life to building canals. The Manchester-Liverpool ship canal nearly ruined him, but ended up bringing in £80,000 a year.
In 1797, at the age of 61, the old bachelor - who would employ no female servants, wore an untidy brown coat and smoked a navvy's clay pipe in public - brought off the picture purchase of the century. He put up £43,000 to aquire 305 paintings from the Orlans collection, which had recently been rescued from revolutionary France.
It had been formed by Louis XIV's nephew - Philippe, Duc d'Orlans - who acted as Regent of France during Louis XV's minority. According to the 19th-century art historian William Buchanan, "the whole collection was formed upon the broad and liberal view of rendering it one of the most splendid and consequential in Europe." With his royal connections, Philippe succeeded in this endeavour.
But the 1790s saw a glut of pictures arriving in London from France, so Bridgewater, the wily old canal builder, got the French and Italian paintings from the Orlans collection for a song. He persuaded his nephew Lord Gower (later Marquis of Stafford and Duke of Sutherland) and the Marquess of Carlisle to come in with him. They kept the 94 best works for themselves and auctioned off the the rest for £42,500, only £500 less than the whole collection had cost in the first place. Bridgewater's deal was regarded by his contemporaries as a national victory, and a compensation for the rankling loss of the Houghton picture collection to Catherine the Great of Russia in 1779.
Bridgewater managed to buy like crazy over the last seven years of his life, and had 250 paintings by the time he died in 1803. The collection was increased by his heir, the Marquis of Stafford, and again by Stafford's son, the 1st Earl of Ellesmere. The number of separate titles enjoyed by this noble family is bewildering, but was greatly simplified in 1963 when they were finally all combined in the person of the present Duke of Sutherland - who is also Baron Gower, Marquis of Stafford and Earl of Ellesmere.
Born in 1915, the present Duke parachuted into Crete in 1941 and spent four years in a succession of German prison camps. He returned thankfully to the family estates at Mertoun in Berwickshire in 1945, and has devoted himself to putting them into apple-pie order. The farmland, woodlands, garden and stud are loved and watched over daily; he demolished two 19th- century wings (and 40 rooms) attached to the 18th-century house to make it into a manageable size, and the paintings he keeps at home have been cleaned and preserved.
The post-war years have seen some reduction of the family's extraordinary art holdings. The bombing of Bridgewater House in London with its purpose- built public art gallery, led to its sale. While the house is now owned by a shipping company, you can still read the inscription "Picture Gallery" on one brass bell-push in Little St James's Street, and "Coachman Grooms" on another.
In October 1946, Christie's auctioned 180 of the gallery's paintings, 32 of which came from the Orlans collection, for a derisory £13,038 18s - they would be worth £13m or more today. The top price was £1,995 for Veronese's Venus Lamenting the Death of Adonis; an Annibale Carracci Vision of St Francis, now in the National Gallery of Canada, made £23 2s ... and so on, and so on. There was little money available for luxuries such as art in the aftermath of a devastating war.
In 1972 it was the turn of Old Master drawings, which are notoriously difficult to conserve and must be kept away from the light. A group of Carracci drawings was sold by Sotheby's for £472,160, and a group by Giulio Romano, Raphael's most brilliant pupil, for £184,010. In 1976 a small group of distinguished paintings was consigned to Christie's, including Turner's view of Dutch fishing boats in a gale, which was commissioned by the Duke of Bridgewater and has been known ever since as The Bridgewater Sea Piece. It made £340,000 and is now on loan to the London National Gallery from an unnamed private collector.
Finally, in 1984, four of the paintings that had been on loan to Edinburgh since 1945 - a Gerard Dou, a Lorenzo Lotto, a Jan Steen and a Tintoretto - were sold to the Gallery for a tax-exempt £2m. The money went into a maintenance fund for Mertoun, the family estate in Berwickshire, Francis Egerton explains.
The struggle to maintain ancestral estates, stately homes and their contents has led to the gradual erosion of most of Britain's old collections. The Sutherlands, who own not only Mertoun but the Stetchworth estate outside Newmarket, currently run by Francis Egerton, share these problems. But they have so far managed to keep Bridgewater's collection remarkably intact. "I would hate to fail after previous generations had managed to hang on to them," says Egerton. "I will try hard not to be the one who breaks the chain."
Tax changes, he emphasises, are the chief threat to the collection. "I would love to be able to be categoric about the paintings' future," he says, "but people are always moving the goal posts - that's what makes one hesitant to lay down hard and fast intentions." Until 9 April, art lovers in London as well as in Edinburgh can see what is at stake; the Duke's Seven Sacraments by Poussin are on view at the Royal Academy. These seven paintings alone must be worth a good £70m. !