Reynolds animated and invigorated the English eighteenth-century portrait tradition, taking it out into the open air and giving it a new drama and immediacy. His work delighted an aristocratic clientele who had grown wearily accustomed to being depicted, by phizmongers of a lesser order, as stiff and glassy-eyed tailors’ dummies in the aquarium light of a perpetual, musty indoors. His prices were unprecedentedly high, his account book for the 1780s showing that the Earl of Bute paid all of £150 for the painting reproduced here. But it was money well spent. Reynolds may never have painted a better whole-length portrait than this. The picture has the drama of a chance encounter. Its effects of movement and instantaneity are cleverly enhanced by the painter’s cropping of the sitter’s dress and by his slightly blurred, impressionistic depiction of the fawning canine projectile that is her dog. It is as if they have both suddenly arrived on the scene. Reynolds does not simply depict his sitter, he puts her into a dramatic relationship with the viewer, who is cast in the role of the houseguest suddenly bumping into his hostess somewhere in the garden. But the painting is edgy and uncomfortable, as well as extremely vivid. The Countess seems less than delighted to have met with us. Her expression suggests that she might rather be alone.

When Reynolds painted the picture he was turning over in his mind many of the challenges of portrait painting. The Countess of Bute began sitting to him in 1777, and in 1778, while her canvas was still on his easel, he delivered the eighth of his magisterial Discourses on Art to the students of the fledgling Royal Academy. One of the problems that he addressed on that occasion was how best “to add Grace and Dignity to the characters of those whose pictures we draw”. He held up the portraits of the Italian Renaissance master Titian as an example, praising their “unaffected air … where dignity, seeming to be natural and inherent, draws spontaneous reverence, and instead of being vainly assumed, has the appearance of an unalienable adjunct.”

Perhaps the rather curious, spontaneously forbidding expression Reynolds gave to the Countess of Bute is explained by this passage. It may well be that it was his way of attempting to give her “dignity” while making it seem “inherent… an inalienable adjunct” – thus bathing her in the same aura of natural, unforced authority that he admired in Titian’s portraits. But I cannot help feeling that there is more to it than that. If she were simply “dignified” she would not be so memorable. While there is a certain haughtiness and pride in her eyes, she also walks with a slight but unmistakeable stoop and has an air of burdened anxiety. The picture leaves the strong impression that Reynolds has told us rather more of the truth about his sitter than was strictly required by his commission. As the nineteenth century author C.R. Leslie commented in admiration of the work, it is “a proof how little inclined the painter was, sometimes, to sacrifice character to beauty.”

The Countess of Bute has been remembered primarily because of those to whom she was related. When she was a young woman she fell in love with John Stuart, the young, handsome but not especially wealthy Earl of Bute, and married him despite the strenuous objections of her parents. Their qualms disappeared when the Prince of Wales, the future George III, took a shine to their new son-in-law, who thanks to his royal patron eventually rose to become Prime Minister (albeit a very unpopular and mercilessly satirised one). Having attained such dizzy heights of social eminence the Countess of Bute spent a considerable portion of the rest of her life attempting to contain the dreadful embarrassment caused her by members of her own family. Her brother was a spendthrift bigamist who wandered the continent marrying and remarrying for money. That was bad enough, for the wife of a Prime Minister. But the real thorn in her side was her mother, Lady Mary Wortley Montagu, who was one of the most colourful and talked-about bluestockings of the day.

Much to the mortification of her ever-increasingly straitlaced daughter, this remarkable woman lived almost completely outside the conventions of her time. During the early years of her marriage to Edward Wortley Montagu, British Ambassador to Turkey, she travelled to Constantinople where – unlike most European wives – she showed a deep interest in indigenous Turkish culture. She visited mosques and infiltrated harems, developing great sympathy and admiration for the women who passed their lives there. Discovering that the Turks inoculated for smallpox, she brought the practice to England, thus pioneering the development of effective modern vaccination. Later she separated from her husband and passed much of her time in Italy, whence she had gone in vain pursuit of a lover, and from where she continued to alarm her daughter by writing satirical verse and witty proto-feminist tracts. In her frail old age she made matters still worse by returning to London, where her curious clothes, unusual ideas and fascinating past attracted an enormous amount of attention. Her posthumously published Embassy Letters, in which she described her early experience in Constantinople, became a huge bestseller. Samuel Johnson said it was the first volume he turned to when he wanted to read simply for pleasure; Voltaire said it was a book written for all times and all nations; while Edward Gibbon, after spending all night perusing it, exclaimed “What fire, what ease, what knowledge of Europe and Asia!”

If Mary, Countess of Bute had had her way the Embassy Letters would have been destroyed – together with all the other unpublished letters, journals and other writings by her mother which she did tragically manage to burn almost as soon as the grandest old woman of eighteenth-century letters, was dead. The Countess of Bute brought up her own daughters (she had six, as well as five sons) to detest learning and not to give a minute’s credence to dangerous and foolish ideas about intellectual self-advancement. If any of them did pick up a book she would be warned that she would “turn out like grandma”, as one of them later recalled: “It was this reproach that first informed me I ever had a grandmother, and I am sure I heartily hated her name. Whatever I wanted to learn, everybody was up in arms to oppose it…”