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This week Donald Macleod explores the miraculous chamber music of Mozart's Vienna years. Today, a late masterpiece; and music for an instrument said to drive its performers insane.

In 1791, a blind musician called Maria Anna Antonia Kirchgessner came to Vienna on the latest leg of a long-running tour of Europe. She was then one of the leading virtuosi on her instrument - the glass harmonica. She must have commissioned Mozart to write a piece for her, because he took time out from work on The Magic Flute to produce an ethereal Adagio and Rondo for glass harmonica, flute, oboe, viola and cello - thereby making a valuable addition to the repertoire of music for the family of 'autophone rubbed instruments'. The claims of insanity may not have been entirely without foundation: the glass used contained 40% lead, so lead poisoning must have been a real danger. Another now-defunct instrument prompted Mozart to compose a work that has secured itself a much firmer footing in the repertoire: his Clarinet Quintet in A, K 581 - or, as it should perhaps be known, Basset Clarinet Quintet in A. The basset clarinet was devised by Mozart's friend and fellow-mason the clarinettist Anton Stadler in collaboration with an instrument-maker called Theodor Lotz. Essentially a regular clarinet with a downwards extension of range, it survived - just - into the 19th century before going into a long spell of retirement, until its revival in the 1950s for a performance of Mozart's Clarinet Concerto, which was also written for the instrument's basset variety.

Larghetto in B flat for piano and wind quintet, K 452a
Mitsuko Uchida (piano)
Neil Black (oboe)
Thea King (clarinet)
Julian Farrell (basset horn)
Robin O'Neill (bassoon)

Adagio and Rondo in C for glass harmonica, flute, oboe, viola and cello, K 617
Bruno Hoffmann, glass harmonica
Aurèle Nicolet (flute)
Heinz Holliger (oboe)
Karl Schouten (viola)
Jean Decroos (cello)

Clarinet Quintet in A major, K. 581
Thea King (basset clarinet)
Gabrieli String Quartet.