If this is Bach's treatment of a comparatively small and specialized art-form, it is obviously impossible to reduce the scantiest account of the rest of his work into practical limits here, nor is there as yet a sufficient body of accepted criticism of Bach for such an account to carry further conviction than an expression of individual opinion. Fortunately, however, Bach was constantly re-arranging his own compositions; indeed he evidently regards adaptability to fresh environment as the test of his finest work: and we cannot do better than review the evidence thus given to us, - evidence which only Beethoven's sketch-books surpass in significance.

2. The successful transplanting of a work of art to a fresh environment is obviously a convincing test of our definitions of the art-forms concerned, if only we take care to distinguish between the alterations produced by the change of environment and those that imply the composer's dissatisfaction with the original version. In Bach's case this seldom causes much difficulty; his methods of adaptation are so logical and so varied as to form a scheme of musical morphology with all the interest and none of the imperfections of the geological record; and the few cases in which a work owes its changes to the need for improvement as well as adaptation cause no confusion, but rather form a link between the pure adaptations and the numerous revisions of his favourite works without change of medium. There is, for example, no difficulty in separating the element of corrective criticism from that of the impulse to give an already successful composition a larger or more permanent form, in such cases as the transformations undergone by the movements of the birthday cantata, Was mir behagt ist nur die muntre Jagd, during their distribution among the church cantatas, Also hat Gott die Welt geliebt and Man singet mit Freuden vom Sieg. The fine bass aria, "Ein Fürst ist seines Landes Pan," was obviously ill-proportioned, with its breakneck return to the tonic and its perfunctory close; and Bach's chief concern in adapting it for its place as the aria, "Du bist geboren mir zu Gute," in Also hat Gott, was to remedy this defect.

On the other hand, the use of the delightful ritornello for violoncello from the little aria, "Weil die wollenreichen Heerden," in the birthday cantata, and the restoration of the rejected long instrumental fugato that was to follow, were obviously brought about by the conception of the entirely new material for the voice in the famous aria, "Mein gläubiges Herze." And when the last chorus of Was mir behagt became the first chorus of Man singet mit Freuden, it was expanded to the proportions necessary for a triumphant opening (as distinguished from a cheerful finale) by the adroit insertion of new material between every joint in the design. This material, being new, could not produce the effect of diffuseness that would result from the expansion of the old material already complete in its simplest form, and thus this instance does not imply criticism.

A highly interesting example of pure self-criticism is the Passion according to St John, which was twice revised, and each time reduced to a smaller scale by the omission of some of its finest numbers. The final result was a work of perfect proportions, and of the rejected numbers one (a magnificent aria with chorale) remained unused, two were replaced by finer substitutes, others took shape as one of the most complete and remarkable of the church cantatas, Du wahrer Gott, while the greatest of the figured chorales was transferred to the Passion according to St Matthew, of which it now crowns the first part.

3. Such instances of self-criticism might be paralleled in the works of other composers; but there is no parallel in music to Bach's power of reproducing already perfect works in different media. Here Bach reveals to us identities in difference which we should otherwise never have suspected. Of course it is possible to arrange works in different ways without illustrating any profound identities at all. Handel, for instance, collected several of his favourite choruses in an enormous instrumental concerto (see vol. 46 of the Händel-Gesellschaft), and the result in the case of a chorus like "Lift up your Heads" was ridiculous. Bach, however, does not arrange old work merely to please a court where it was already admired. He never leaves it in a state of mere make-shift, though he cannot always attain his evident aim of a new originality. His methods of orchestration and the profoundly significant identity of certain forms of chorus with certain concerto forms may better be described under their proper headings (see articles Instrumentation and Concerto). Here we will attempt first to show, by illustrations of Bach's power of adding parts to already complete harmonic and contrapuntal schemes, what was his conception of the nature of an art-form, and secondly, by means of a short analysis of cases in which he adapts the same music to different words, to define his range of expression.

Bach arranged all his violin concertos for clavier, including two that are lost in the original version. Here his power of providing new and apparently necessary material for the left hand of the cembalist (or, in the double concertos, two left hands) without disturbing the already complete score, is astonishing; and it fails only in the slow movements, which he prefers to leave obviously in the condition of an arrangement rather than to spoil their broad cantabile style by a too polyphonic bass.

But these cases are insignificant compared with such transformations as that of the prelude of the E major partita for unaccompanied violin into the sinfonia for organ obligato accompanied by full orchestra (including three trumpets and a pair of drums) at the beginning of the church cantata, Wir danken dir, Gott. The original version is perhaps the most complete and natural of the violin solos, for its arpeggios produce full harmony without recourse to that constant attempt to play on all four strings at once, which makes the performance of the polyphonic movements a tour de force in which steady rhythm is nearly impossible. Yet in the sinfonia its proportions seem to reveal themselves for the first time. Not a bar is displaced and not a note of the new accompaniment is unnecessary. The whole is almost entirely without themes; for even this, the largest of all arpeggio-preludes, consists essentially of the gradual unfolding of a scheme of harmony in which rhythmic and melodic organization is reduced to a minimum.