The ability of music to evoke a sense of the ethereal has a strange and powerful effect on listeners, something that composers have been aware of across the ages. Tom Service examines how this music creates its affect and to what ends. He draws on examples from Hldegard of Bingen, Gregorio Allegri, Wolfgang Mozart, James Horner, Einojuhani Rautavaara and George Crumb - among others - and of course Richard Wagner.