Today’s program displays a striking symmetry. Each half opens with a single-movement Baroque composition, contrapuntal and abstract, which serves as a kind of prelude to the large-scale Romantic piece that follows. The pieces are linked by key—minor followed by its relative major—with the G minor of Purcell’s Chacony leading to the B-flat major of Brahms’s Sextet, and, after intermission, the C minor of Bach’s Ricercar leading to the E-flat major of Mendelssohn’s Octet. As we’ll see shortly, the key relationships are not the only connections in evidence.

Henry Purcell’s Chacony is a brilliant example of the chaconne (in its idiosyncratic English spelling), a genre in triple meter characterized by a short musical pattern that repeats throughout the piece, providing a grounded backdrop for the composer’s flights of invention. “Ground,” in fact, was the term the English applied to such patterns. Here, the ground is eight bars long, a simple melody, mostly one repeated note per bar, that descends an octave from G to G with several turns along the way, like the back-and-forth steps of an elegant dance. Purcell generally assigns the ground to the cello, where it provides a bass foundation for the interplay of the upper voices. But he mixes things up in the middle of the piece, twice having the cello drop out while the viola takes over the ground. At another point, he transfers the ground to the highest voice. There are even two passages where the ground disappears entirely. Besides toying with the regularities of the form in these ways, Purcell enlivens the music by varying his rhythms, overlapping the beginnings and ends of phrases, and exploiting the possibilities for expressive dissonance as each of the four voices follows its own melodic path.

The instrumentation of Brahms’s Sextet, Op. 18, with its addition of a viola and cello to the standard string quartet, shifts the weight toward the bass. The first movement pulls us immediately into that luxurious sound world, with the noble opening theme given to cello, accompanied only by the other cello and a viola, playing in their lowest registers. The violins respond, repeating the theme, now doubled in octaves with the viola, changing its register and color. Thus, right from the start, Brahms treats the chamber ensemble as a small orchestra, with its rich possibilities for contrasts of instrumentation. He also primes the listener for a journey of the emotions, introducing flatted notes in the theme that hint at undercurrents of sadness or longing, giving rise to an urgent obbligato line in the violin, and then, like a distant patch of blue sky in the midst of rain, a pianissimo chorale fragment in the glistening key of A major.

The passionate second movement is a theme with variations, a genre in the same family as the chaconne, but, as was more typical in the Classical and Romantic periods, with a theme in two parts, each of which is repeated, AABB, providing the underlying harmonic and formal structure for the variations that follow. Interestingly, the bass line of Brahms’s theme is remarkably similar to that of the Chacony in its overall shape and several of its melodic turns. The third movement scherzo begins as a delightful folk dance, good-natured and gently syncopated. Then it picks up speed in the middle trio section, clogs stomping and skirts swirling. A return to the opening dance is followed once again by a fragment of the trio, this time in a quick dash to the end. The opening of the last movement parallels that of the first movement, with Cello I taking the lead, accompanied by Cello II and one of the violas. After they have presented the graceful rondo theme, the other three instruments repeat the theme an octave higher, a call-and-response suggesting a dialogue between two characters. For Brahms, who never wrote an opera, this kind of instrumental writing was a way of giving voice to his powerful dramatic urges, without the need for words to convey the ineffable emotions of human interactions.

The pianist and theorist Charles Rosen described Bach’s Ricercar a 6 as “the most significant piano work of the millennium … at least, the first piece that a composer knew would certainly be played on a piano.” Bach would have had in mind one of the recently-invented Silbermann pianos installed at the Potsdam estate of Frederick the Great, instruments he had tried out on a visit in 1747 at the invitation of the king. While there, Bach also displayed his skills as a performer and improviser, creating a three-voice fugue on the spot with a theme the king had given him. On his return to Leipzig, Bach used the same theme to write a small collection of pieces, adding a six-voice fugue (the “Ricercar a 6”) to his improvised three-voice fugue, as well as a multi-movement trio sonata and several canons. He had the collection engraved and printed and sent the package off to the king as “A Musical Offering.” It is in the nature of such abstract music that it works well when transcribed to other instruments, as in the version we hear today. Since the opening of a fugue requires that each voice enter one by one, presenting the theme in full before the next voice takes its turn, the sixth voice, played by cello, must wait nearly two minutes for its entrance.

The music and spirit of Bach permeated the household where the child prodigies Fanny and Felix Mendelssohn grew up. The children played and sang Bach from an early age, while developing their compositional skills through regular assignments from their tutors in Bachian harmony and counterpoint. They were also well-steeped in the music of Handel, Mozart, Haydn, Beethoven, Weber, etc. Given all this, and his own gifts and voracious intellectual appetite, it is not surprising that Felix, by 1825 when he was 16, had already produced several enduring masterworks, culminating in October with the Octet, a piece that has come to be acknowledged as one of the pinnacles of the chamber music repertoire.

Was it the audacity of youth that prompted Mendelssohn to write a piece of chamber music for so many players? Perhaps he was inspired by Louis Spohr’s several works for double string quartet, perhaps by Bach’s instrumental writing, as in the nine string parts of Brandenburg Concerto No. 3. Mendelssohn deploys his forces with great variety and ingenuity. He might divide the group into various smaller configurations for antiphonal passages. Or, at climaxes, have all eight players join in octaves to play the same material. The opening suggests a concerto, with solo violin playing a rising arpeggio line supported by shimmering harmonies from the rest of the ensemble. But then the two cellos take over in the bass, answered by shouts from two violins.

The cellos and violas open the second movement with a tragic tune in C minor. Almost immediately, the four violins reach out to offer consolation in the unexpected key of D-flat major. These emotional tugs in different directions give rise to music of passion, poignance, anger, serenity. The following movement, an uncanny scherzo, is marked to be played “always pianissimo and staccato.” Here, we enter a realm of sprites and spirits, emissaries from the darkness. The final bars evaporate like a wisp of fog, setting us up for the shocking opening of the last movement—a single cello, low and loud, fast and furious, playing what sounds like the start of a fugue. One by one, the eight instruments quickly pick up the theme. However, once they are all in, they abandon the notion of a fugue and join in a unison chorus to present a new theme. The music is full of such sudden shifts, a virtuosic showpiece of youthful exuberance, including, near the end, a brief reprise of music from the scherzo.