It was the opening night of the Library of Congress' annual Festival of American Chamber Music. Intermission was just over during an 18th-century English ballad opera called "The Mock Doctor, or The Dumb Lady Cur'd," by Henry Fielding, no less -- and based on Molie re.

Dorcas, the plump, buxom milkmaid character (sung by Nancy Armstrong), entered from the rear of Coolidge Auditorium, floating her baroque roulades and runs through the spaces of the hall as she descended the steps toward the stage, flirting with the men on the aisle seats. About two-thirds of the way down, something drove her to go a degree farther -- suddenly planting herself in the lap of a hapless music critic as she continued her rigorous coloratura displays.

This is the stuff of ballad opera -- the lusty, wildly satirical art form that lived a brief but brilliant decade or so on the London stage in the 1720s and 1730s, reaching its peak in John Gay's classic, "The Beggar's Opera."

These works became just as popular in the colonies (which "The Mock Doctor" reached in 1750), so it only fitting that their torch is carried today by the Boston-based group, the Friends of Dr. Burney (the eminent music critic), who brought this two-hour frolic to the normally sober confines of the Library.

Such works are eclectic grab bags without compare. Most of the numbers are lifted from other composers. Three last night came from "The Beggar's Opera," which did its own share of stealing from elsewhere. One of the trios between the would-be doctor and the master, Sir Jasper's two servants, is set to the theme of "Drink to Me Only With Thine Eyes." Another uses the music of "Glory, Glory, Hallelujah!" There are wonderful parodies of the Handel operas, which ballad opera displaced on the London stage (otherwise, we never would have had "Messiah").

This group of joyful musicians, under Charlotte Kaufman, constitutes yet another limb of the burgeoning authentic performance movement; without their deftness, one might think Fielding's memory was better served by exclusive concentration on "Tom Jones." One cannot single out all the best ones, but David Ripley, as the phony doctor, was especially marvelous.

A final thought: Several weeks of repertory by the Friends would be a real feather in Peter Sellars' cap at the Kennedy Center's Terrace Theater.