The opera shifts from its pastoral and sensual mood into an elegiac quality as the chorus warns Acis and Galatea about the arrival of a monstrous giant, Polyphemus, singing "no joy shall last". The fugal minor-key of the chorus's music along with the percussive lines in the lower instruments, indicating the heavy footsteps of the giant, provides an effective dramatic transition into the more serious nature of the second act. Polyphemus enters singing of his jealous love for Galatea, "I rage, I melt, I burn", which is in a part-comic furioso accompanied recitative. This is followed by his aria "O ruddier than the cherry" which is written in counterpoint to a sopranino recorder. Polyphemus threatens force but is somewhat soothed by the impartial shepherd, Coridon ("Would you gain the tender creature"). Meanwhile, Acis ignores Damon's warning of the fleeting existence of love's delight ("Consider, fond shepherd") and responds with hostility and the determination to resist ("Love sounds th' alarm"). Acis and Galatea promise eternal fidelity to each other in what begins as a duet ("The flocks shall leave the mountains") but ultimately turns into a trio when Polyphemus intrudes and in a rage murders Acis. Galatea, along with the chorus, mourns the loss of her love ("Must I my Acis still bemoan"). The chorus reminds her of her divinity and that with her powers she can transform Acis's body into a beautiful fountain. The work closes with Galatea's larghetto air, "Heart, the seat of soft delight", in which she exerts her powers to enact the transformation, ending with the chorus celebrating Acis's immortalisation.
