They were fast friends: the 41-year-old French beauty, wife of a rich civil servant, and the 79-year-old Benjamin Franklin.

He had been in Paris for nine years, ever since he helped write the Declaration of Independence. She was his neighbor in fashionable Passy, a talented harpsichord player, a composer who knew Boccherini and Johann Christian Bach, a chess player.

He wrote 126 letters to Mme. Brillon de Jouy that we know of ... in French. He had long since taught himself French, Spanish, Italian and Latin. Frequently he apologizes for his clumsiness; charmed, she replies that if his French "is not very pure, it is at least very clear!"

He flirted with her, and she, married to a man 24 years her senior, responded with warmth.

"Do you know, my dear papa," she wrote, "that people have criticized the sweet habit I have taken of sitting on your lap, and your habit of soliciting from me what I always refuse?"

Then, in 1785, he was recalled to America. He left on July 12 in one of "the King's litters, carried by mules," headed for the coast. But not before she had arranged for a souvenir, a portrait by the court artist Joseph Duplessis.

Just a few weeks ago the painting, which has been in the hands of the Brillon descendants for more than two centuries, was acquired by the National Portrait Gallery through a grant from the Cafritz Foundation and presented to the public for the first time.

It is a superb portrait. The familiar face looks right at you: those old bloodhound eyes, kind but sharply observant, the mouth held closed as though by effort, the long scraggly gray hair and tremendous forehead ...

"There are several versions of it," commented gallery director Alan Fern, "but I think this may be the first oil impression. It has a freshness, a directness, the face is so expressive."

Franklin wore no powdered wig, of course. He never did. He had on his favorite old gray coat for the occasion and a rather wrinkled stock that did nothing for his aging throat. It was this simplicity, combined with his gentle wit and enormous learning, that endeared him to the French, who were, one recalls, trying to live through the rococo age of Louis XVI.

And he loved them too, especially the women. The old widower kept up with many Frenchwomen, most of all with his pretty neighbor, whose inquiring mind must have been a delight to this wrinkled child of the Enlightenment. Mme. Brillon introduced him to the aquatint, a new technique being developed by Paris artists. He composed some music for her and presumably played it on the glass harmonica he invented. He also wrote several bagatelles, elegant little prose essays, for her.

A lovely intimacy. They met on Wednesdays and Saturdays. They discussed how they would meet in Heaven one day and would have little concerts and "eat apples of Paradise roasted with butter and nutmeg. And we shall pity those who are not dead."

As for that tiresome 1980s question: Did they go to bed? Surely not.

"In salon lovemaking," reports Bruce Ingham Granger in his biography of Franklin, "the man respected his adversary just so far as she knew how to parry his every thrust; always it was hoped that the contest would end in a draw ... Many Americans, however, unaware of ... the conventions of the salon, misconstrued the meaning of these letters, as they did his behavior in the presence of the ladies and certain of his reported conversations in France, and thus encouraged the growth of the image of Franklin the philanderer."

Ben Franklin loved women, all right, but how much? Go take a look at his face.