The sun rises. Dante and Virgil, still on the shore, see a white light moving impossibly fast across the sea — too bright, too swift for any natural ship. As it nears, the wings of an angel become visible, vast and radiant, serving as sail and oar at once. This is the angel boatman, servant of God, who ferries souls from the mouth of the Tiber (where the righteous dead gather) to the base of Mount Purgatory. The contrast with Charon's grim ferry across the Acheron is deliberate and total.
The souls disembark, singing the 114th Psalm: In exitu Israel de Aegypto — "When Israel came out of Egypt." The choice is deeply Dantean: the Exodus from Egypt is the paradigm of liberation, of a people leaving the house of bondage for the promised land. Dante himself, in his famous letter to Can Grande, cites this very psalm as the key to understanding the allegory of the whole Comedy. The souls are not prisoners like the damned; they arrive singing.
Among the new arrivals, Dante recognizes his old friend Casella, a Florentine musician who had set some of Dante's early poems — most likely his canzone Amor che nella mente mi ragiona — to music. The reunion is warm and moving. Dante tries twice to embrace Casella, and twice his arms close on air — the shades have no substance. He asks Casella to sing, and Casella obliges, performing the very canzone. The souls gather around, enchanted, forgetting their climb.
Cato's voice breaks the spell. He storms at the lingering souls with harsh reproach: why do they tarry? Why has this song made them forget their task? The souls scatter toward the mountain like startled pigeons, and Dante and Virgil follow. The moment captures one of Purgatorio's central tensions: the legitimate beauty of earthly art, friendship, and music versus the urgency of the soul's work of purgation. Pleasure must not become a new delay.