The Handel and Haydn Society Chorus. (Stu Rosner/Handel and Haydn Society)

When the Handel and Haydn Society was founded, its name indicated that the group focused on new music (Haydn) and old (Handel). Now, 200 years later, the name indicates a focus on the past for this period-instrument ensemble and chorus that remains the oldest continuously performing ensemble in the United States. On Saturday, the Boston-based group came to the Library of Congress to show both sides of its history: a program that offset baroque and classical music with the Washington premiere of a new work.

In lieu of the society’s full forces, Washington got a small group of instruments — a string quintet and organ player Ian Watson — and a chamber chorus. It also got a program that was elegantly conceived if not always elegantly performed. This wasn’t for lack of effort on the part of Harry Christophers, the group’s artistic director since 2009, who had an engaging way of sometimes striding around the stage as he conducted, the better to focus on particular musicians and particular areas.

Although sometimes a little sloppy, the chorus included some very good soloists. A focus of the first half were three selections from an anthology that the society published in 1823, including an arrangement of Mozart’s aria “O Isis und Osiris,” from “The Magic Flute,” here rendered as “Almighty God, When Round Thy Shrine”; they offered showcases for, in particular, the soprano Margot Rood, colorful and vital, and the rich, clear contralto Emily Marvosh. On the second half, a trio of pieces by William Byrd were a highlight of the program, starting with “Ye Sacred Muses,” an elegy written after the death of the composer Thomas Tallis, sung with sensitivity and beauty by the tenor Stefan Reed.

The instrumentalists were not as strong. Two instrumental works by Purcell, a Chacony and a Pavane (both in G minor), showed the two violinists and viola to poor advantage, sawing and wandering.

They wandered, too, in the new work, “My Angel, His Name Is Freedom” by Gabriela Lena Frank, co-commissioned by the society and the Library of Congress. This attractive piece marries hints of a simple folk sensibility with a sophisticated sense of musical structure to create something bluntly imposing, trumpeting out a text by Ralph Waldo Emerson.

The chorus, though, saved its real impact for the final Bach motet, “Singet dem Herrn ein Neues Lied,” BWV 225, which zipped and rollicked and made a sharp finish to a sometimes-blurry evening.