Besides these there are the three great independent toccatas and the Passacaglia. The remaining choral-preludes fill one Jahrgang, and the other organ works two more.
D. Unclassified
Two important instrumental works cannot be classified, viz. Das musikalische Opfer, the volume of compositions (two great fugues, various puzzle-canons, and a splendid trio for flute, violin and figured bass) on the theme given to Bach by Frederick the Great; and Die Kunst der Fuge, a progressive series of fugues on one and the same subject, written in open score as if entirely abstract studies, but all (except the extreme contrapuntal tours de force) in admirable clavier style and of great musical value.
IV.—Lost Works
A. Choral
J. N. Forkel's statement that Bach wrote 5 Jahrgange of church cantatas (i.e. enough to provide one for each Sunday and holy day for five years) would indicate that some 80 are lost, but there is reason to believe that this is a great exaggeration. Not more than six or seven cantatas are known to be lost, by the evidence of fragments, text-books, &c.
Forkel also says that Bach wrote five Passions. Besides the great Matthew and John Passions there is in an indisputable Bach autograph one according to St Luke; but it is so worthless that the best plea for its authenticity offered by responsible critics is that only a personal interest could have induced Bach to make a copy of it.
The lost Passion according to St Mark must, judging by the movements preserved in the Trauer-Ode, have been larger than that according to St John.
Was there a genuine Lucas-Passion? If so, Forkel's report of five Passions would be explained. Several lost secular works are partly preserved in those portions of the Christmas oratorio of which the sources are not definitely known, but which, like the other duplicated numbers, are fair copies in the autograph.