This is the aria Mary sings as they walk to the lake that night, also known as Handel’s Largo: a simple, exquisite ode to a plane tree. It’s from Serse, an utter flop of an opera that premiered in 1738 and was performed a whopping five times in London, with Caffarelli playing Serse/Xerxes. Mary has a thing for Handel.
Besides the delightful theatricality of this video, I love that it’s Bartoli singing: her album Sacrificium is on regular rotation here, a celebration of music written for the specific instrument that is the castrato voice. (Note too her interesting choice of art for the album.)
To hear the full range of a castrato, however, the closest we can get right now, I think, is the movie Farinelli, an extremely loose biopic of the famed singer. To create the range of his vocalization (his voice ranged from tenor to soprano) they stitched together two singers, a soprano and a counter-tenor. The plot veers as far from fact as dramatic license allows, but the music is exquisite.