Of the seven movements of this cantata, I found myself most interested in the vocal texturing of the second movement, the soprano-alto duet. As is his wont in church music, Bach utilizes the nuances of the form, in this case the similarities and differences between the two vocal lines, to underline and highlight the sacred nature of the text. These nuances are comprised of four main types of texture: a canon between the voices, identical rhythms with sixths and inverted sixths (thirds) between the voices, a somewhat canonical rhythmic trade between the voices which occurs only in the first section of the piece, and finally a freer contrapuntal texture which occurs only in the second section of the piece.
Before I delve into the relationships of textures to text, I would like to define the sections of the second movement with a view to improving clarity. From the beginning of the piece to m.50 (the fermata) is the First Section. From m.50 to the end of the piece is the Second Section. The first section is made up of the First Ritornello (m.1-8), the A Section (m.9-21), the A1 Section m.22-29), the A2 Section (m.30-42), and ends with a statement of the Second Ritornello (m.43-50). The second section starts with the B Section (m.50-60), has a modified Third Ritornello (m.61-64), the Development Section (m.65-80) which could also be called B1 because of its close relationship with B, a Short Ritornello (m.81-82), the B2 Section (m.83-94), and the Closing Section (m.95-da capo) which is reminiscent of A2. These terms are rather arbitrary and are intended almost exclusively to enable easy discussion and comparison of the parts of this movement.
The first texture is a canon which begins most of the sections, in which rhythms are passed between the voices. This is most evident at m.8, m.20, and m.30 (which is really somewhat of a repetition of m.8). M.8 introduces what will be the main melodic theme of the movement, an eighth and sixteenth-note rhythm. The canon at m.20 acts as the beginning of A1 and the canon at m.30 is the beginning of A2. It is well to note that both of these extremely `busy' bits of melody are ornamentations of the word "hasten." As such, they both hasten the rhythmic movement of the piece and reinforce the melodic theme. After the fermata in m.50 the canon enters again, heralding the B section. This canon is, appropriately for a developing idea, based on a different melody, which bears great similarity to m.25 and m.33. This explication of previous material is somewhat of a Bachian formula, and perhaps does not warrant close scrutiny, but it seems that both uses of this melodic material relate to physical movement. In m.25 and m.33 the text speaks of "weak but persistent steps" and in m.50 the lyric is "faithfully seek the sick and the wandering." The canon does not reappear until m.69 in the alto, and then shows up again in m.83. The canon in m.69, a rising melodic line which is repeated a fourth up when the soprano enters, is significantly on the words "raise our voices." The canon in m.85, which contains elements of both the A section and B section canons, is the first appearance of the text "Es sei uns dein," and represents a new idea: the successful completion of the Christian sinner's search.
The next textural structures are the sixths in identical rhythm between the two voices. This occurs at m.15, m.32, and for the last time at m.92. I postulate two reasons for the use of parallel sixths. The first is that, as a consonant interval in unison rhythm, the text which is set to the sixths is quite easily heard, more so than the sometimes confusing interplay of voices in the canon and especially in the free counterpoint. The second is that the purely textural contrast between sections with parallel sixths and canons are a very effective way to maintain the listener's interest. At m.15 the line "zu helfen zu dir" is introduced for the first time, but other than that the sixths are used only to restate earlier material, and these is no thread of continuity relating the subject matter of the text so treated.
Far more useful to Bach is the inversion of the sixth, the third. I believe that thirds between the voices are the meat and potatoes of this piece. They appear in m.27, the second time through "zu helfen zu dir," as a counterpart to the sixths of m.15. And they are quite prominent in m.38, the tag end of A2 and a definite precursor to the canon of m.50, the B section. The thirds come into their own at the most dramatic moment of the piece, the "Ach hore" of m.64. At this climactic point they dominate for five measures, and then disappear into a sea of free counterpoint until m.87. In other words, the period in which the congregation "raises their voices to pray for help" contains no thirds, but they reappear when Jesus' "gracious countenance cheers" them. This is a conclusive link between the musical and lyrical ideas of consonance. As closure, the final appearance of this textural device is in m.95, as the point is further made with a repeat of m.87. These sixths and, more prominently, thirds are used by Bach as a congruous means of resolving the tension of his more independent and contrapuntal sections.
The third main texture is the somewhat canonical trade of the "O Jesu, o meister" lines in m.16 and m.28, and the similar trade on "zu dir" in m.36. This construct only occurs in the first section of the piece which, with the da capo included, makes a total of six occurrences. I think this texture is mainly just a nice way to move the aria along and still highlight the importance of the call to Jesus. Also, its escalating nature suggests that the music is reaching up to the heavens. The first two instances of this texture end their respective sections with this call to heaven, and the "zu dir" in m.36 is definitely a very important concept in the lyrical aspect of the work, completing as it does the pattern of A, A1, and A2, each bounded by a ritornello in front and this vocal texture at the end.
The relatively free contrapuntal section begins in m.55. There is restatement of material from this section, notably in m.75, in which the alto takes the part which was sung in the soprano in m.55. This section is also remarkable for its heavy dependence on contrary motion as a means of conveying musical movement. Altogether, however, this is the most freely composed and least formulaic section. The text, as has been explained, is concerned at this point with "raising voices" to pray to Jesus, and the musical idea of rhythmic complexity combined with little recognizable restatement captures quite nicely the image and sound of many voices crying out to their God. This subdivision also includes the parallel fourths in the last beat of m.58 through m.59. I submit that these fourths, a dissonant interval in the practice of the day, are intended to set off the powerful and dramatic consonance of m.65. They appear only once in the piece because m.80 is followed by a ritornello which precludes the final segment, and there is no need for a blatant texture. M.80 is the end marker of this freer section, as the canon and the sixths/thirds once more dominate both the closing section and the eventual da capo .
Bach's most masterful use of texture as a tool toward defining an image probably occurs in the measures from 50-80, but the entire second movement is a tour de force of art inside science. Bach created this 78th cantata to fit the quite formulaic pattern of the Mass as well as glorifying his sacred subject. In a sense he was performing as nothing more than a craftsman filling a customer's order. And yet he was able to transcend the mundanity of his made-to-order piece and truly give, in his way, to the greater glory of his God. Or at least so I assume. It seems that Bach's prodigious technique and sense of musical style are able to coexist as equals, a rare situation. The usual pattern calls for one or the other to dominate an define to limits of a work, but Bach's attributes, as always, complement and sustain each other.