Besides the "Little Organ Book," we possess a second autograph collection by Bach, containing sixteen organ chorals, interspersed with several pieces of the same order written by his pupil Altnikol. In this volume are included the most noteworthy of the more extended organ chorals composed during the Weimar period; these however were not published until they had undergone the careful revision of the artist and received many changes from his hand. Here we find model compositions in all the larger forms, which either came down to Bach from his predecessors, or were acquired by his own continually increasing culture. His constant and energetic efforts to attain the greatest possible organic unity in his compositions naturally obliged him to place the strongest emphasis upon the development of the polyphonic web of the melodic accompaniment from a single musical thought. If this thought became expanded into a longer and more expressive theme, worked out according to the rules of art, the accompaniment was then likely to acquire an independence and importance which interfered with the predominance of the choral melody. The sole means therefore of strengthening this melody was to introduce a vocal part, and that is precisely what was done by Bach. While he added to the composition thus changed, other instruments besides the organ and also other vocal parts, he transferred the organ chorus to the domain of vocal music, in order that he might give it the grandest conceivable form. Bach's choral choruses, which excite the astonished admiration of all who know them, are the direct outgrowth of his organ chorals. The piece from the "Little Organ Book" consisting of twenty-seven measures, composed upon the melody, "O Lamm Gottes, unschuldig" ("O, Innocent Lamb of God"), and the gigantic opening chorus of the St. Matthew Passion, in which the same melody is accompanied and crowned by two vocal choruses, two orchestras and two organs, rest upon exactly the same principle of construction. Buxtehude had paid no attention to the third method of developing the choral, as previously described, but bestowed much thought and artistic skill upon the second, and had succeeded in imparting to it an especial charm, through tasteful ornamentation of the melody and the combination of several different kinds of organ registers. His works in this line, however, are not distinguished by any especial unity or by reserve of form. Bach felt himself attracted by this free and more fantastic style of composition, because it stimulated him to evolve from it something which should satisfy his own lofty ideal. A number of highly remarkable organ chorals, of the most dissimilar character, originated in this way; among them are the elaborations of "Schmücke dich, o liebe Seele" ("Adorn thyself, beloved Soul"), "An Wasserflüssen Babylon," two re-arrangements of "Allein Gott in der Höh sei Ehr" ("Glory alone to God on High"), and one of "Nun komm, der Heiden Heiland."