3. _Fifteen three-part Inventions_, also called Symphonies. They were written for the same purpose as the Inventions, but are more advanced.(257) 4. _The Well-tempered Clavier_, or, Preludes and Fugues in all tones and semitones, composed for the profit and use of young musicians desirous of knowledge, as also for those who are skilled already in this studio. Part I. was finished in 1722. Part II., like Part I., contains twenty-four Preludes and twenty-four Fugues in every key, and was composed at a later period.(258) Every number of it, from first to last, is a masterpiece. In Part I., however, certain Preludes and Fugues bear marks of immaturity and are included probably only in order to complete the series. But here again Bach eventually corrected whatever seemed to him lacking in finish. He altered or rewrote entire passages, so that in the later texts few movements are not perfect. Among these few I reckon the Fugues in A minor,(259) G major and G minor,(260) C major,(261) F major and F minor.(262) The rest are excellent, some of them so superlatively good as to be not inferior to those in Part II.(263) Even Part II., for all its original perfection, has been improved by the composer, as may be observed by comparing the original and later texts. Both Parts contain treasures of art not to be found outside Germany. 5. _Chromatic Fantasia and Fugue._(264) I have taken considerable pains to discover a similar piece of music by Bach, but without success. The Fantasia is unique and unequalled. Wilhelm Friedemann sent it to me from Brunswick inscribed with these words by a mutual friend. “Anbey kommt an etwas Musik von Sebastian, sonst genannt: Fantasia chromatica; bleibt schön in alle Saecula.” It is remarkable that this piece, for all its technical skill, appeals to the most unpractised hearer, if it is performed at all tolerably.