Mendelssohn was briefly between jobs when he wrote the second of his two cello sonatas; he was making the transition from handling musical matters for the King of Prussia to assuming the directorship of the new Leipzig Conservatory. The attendant stress of this period is hardly reflected in this sonata, which the composer wrote for his brother Paul, a cellist. The first movement, Allegro assai vivace, begins with a surging, confident melody for the cello, underpinned by a pressing piano accompaniment. The keyboard then presents its own statement of the theme, the cello now relegated to the background. The piano remains dominant during the introduction of the more lyrical but still impulsive second subject, but the cello eventually gets its turn at an extension of the tune. Unwary listeners might gather that Mendelssohn is here launching a third subject, but the cello is clearly playing the flowing outline of the melody the piano has just broached. Indeed, the instruments will be treated as equals throughout most of this sonata, and often each is allowed a virtuoso flourish. Mendelssohn develops his themes sequentially and neatly, with an almost circumspect ardor that never subordinates one instrument to the other.
The second movement, Allegretto scherzando, opens with a whimsical tune in the piano that is quickly taken up pizzicato by the cello. The melody becomes a bit more lyrical when played arco, but Mendelssohn reserves real songfulness for the second theme, which the cello sings out over a palpitating piano accompaniment. The movement continues with a repeat of the first section that turns uncharacteristically gruff before a brief reprise of the second theme, again in the cello. The instruments quietly slip away, playing fragments of both melodies.
The Adagio begins with graceful piano arpeggios that follow the chord structure of the aria "Es ist vollbracht" from Bach's St. John Passion, reflecting Mendelssohn's lifelong devotion to the music of the German Baroque master. The piano recedes into the background as the cello offers an aria of its own, which grows more ardent and recitative-like as it progresses. Before long, the cello is playing its aria over the piano's chorale-like arpeggios; at the very end, the piano offers its own treatment of the cello's theme. Cellist Coenraad Bloemendal has proposed reading this movement as "a programmatic representation of the competing religious forces that coexisted in Mendelssohn's mind" -- the "Lutheran" piano part overlapping and ultimately embracing what Bloemendal describes as the cello's "cantorial chant."
The extensive finale, Molto allegro e vivace, begins with hectic yet low-key material that recalls the composer's famous Spinning Song. All of this sonata-form movement's thematic elements are spun from the material of the first several measures. Mendelssohn does not vary or develop this material so much as use it to put the two instruments through accelerating, arduous runs, culminating in a final section that creates a dazzling effect without descending to vulgarity.