According to Martin,
Sorge had observed that the dominant ninth chord was taken often without preparation in music, and he sought a natural source for this sonority within the overtone series. . . . With the Compendium Sorge made three important contributions to the field of musical theory regarding seventh and ninth chords: (1) he rejected the theory of supposition as a source for the ninth chord; (2) he recognized the dominant seventh and ninth chords as essential sonorities which could appear unprepared; and (3) he accepted all positions of the bass and offered numerous examples for practice and study. (p. 39)
Here is the charming opening passage of Sorge's Chapter X “On the Free and Unsuspended Ninth, and Its Derivatives” in Martin's translation: “The ninth free and unsuspended? Indeed, yes. Isn't that new? Certainly not. Nature had created it long ago, yet it just wasn’t known. . . . This free and unsuspended ninth ascends above the seventh; thus, it is found over the dominant chord in both [major and minor]. And it decorates this very harmonic structure like a beautiful gold button” (p. 204). His explanatory example has first a major dominant ninth, then a minor ninth.
Later Sorge declares that “If space permitted, I could refer to more than a thousand examples from our great composers of the past. . . . Just a single instance: look at the Duetto in the opera Angelica e Modoro, "Dimmi una volta addio" of our blessed Capellmeister Graun" (p. 205). Here is that example. Clearly Sorge means by "free and unsuspended" not an independent dominant ninth sonority in the 19th-century manner, but a ninth that is struck without the preparation that would be required of a suspension.
In every instance in the two examples above, the resolution (that is, continuation from the ninth) is what I call internal, within the dominant, not external which would be to the next chord.
David Sheldon's article on the ninth chord in 18th century German theory remains an essential resource. He describes Sorge as a "conspicuous and important figure in the history of the
ninth chord" (p. 73). In his earlier theory (that is, before the Compendium), Sorge relied on J. D. Heinichen's categorization of thorough-bass figures to explain the ninth chord as an inversion of a non-triadic chord (!), which Sheldon attributes to Sorge's determined rejection of Rameau's (and Marpurg's) theory of supposition. Once he accepts the dominant seventh chord as a fundamental entity, however, Sorge's thinking changes, leading him
Thus, we begin to move beyond the Ramellian notion of the ninth chord as derived from the dominant seventh by adding a third below the root, only to find an equally awkward removal of the root, an early presentation of a common idea in 19th century theories where the dominant ninth is accepted as a basic chord but understood to appear in practice mainly (or only) as one of the diminished seventh chords. We will see the most extreme form of this notion in Gottfried Weber's treatise.David Sheldon's article on the ninth chord in 18th century German theory remains an essential resource. He describes Sorge as a "conspicuous and important figure in the history of the
ninth chord" (p. 73). In his earlier theory (that is, before the Compendium), Sorge relied on J. D. Heinichen's categorization of thorough-bass figures to explain the ninth chord as an inversion of a non-triadic chord (!), which Sheldon attributes to Sorge's determined rejection of Rameau's (and Marpurg's) theory of supposition. Once he accepts the dominant seventh chord as a fundamental entity, however, Sorge's thinking changes, leading him
to a tentative, yet modern conception of the dominant ninth chord as a fundamental harmonic entity. It is modern in that the ninth is achieved by adding a third above the dominant seventh chord rather than below; the real fundamental bass of the ninth chord is therefore its lowest note, not its third. Sorge attempts to prove the primacy of the dominant 9 chord not only by [inverting it] but also, much more importantly, by regarding it as the basis for the seventh chords on the leading-tone degree of major and minor. Practice, rather than theory, seems to constitute the heart of Sorge's argument here. The fact that neither the ninth nor the seventh of the dominant chord need be prepared, and the fact that the dominant seventh and dominant ninth chords (or any of their inversions) can be substituted for each other can be taken as implicit proof of Sorge's claim regarding the origin of the leading-tone seventh chord. In an entirely new manner, therefore, Sorge uses the concept of a ninth chord to justify free handling of the leading-tone seventh chord. . . . Sorge's acceptance of the V9 is, nevertheless, tentative, applying only to those situations in which the ninth is freely approached. (pp. 73-74)
Reference:
Damian Blättler, "A Voicing-Centered Approach to Additive Harmony Music in France, 1889-1940," PhD dissertation, Yale University, 2013.
James M. Martin, “The Compendium Harmonicum (1760) of Georg Andreas Sorge: A Translation and Critical Commentary,” PhD dissertation, The Catholic University of America, 1981.
David Sheldon, “The Ninth Chord in German Theory,” Journal of Music Theory 26 (1982): 61- 100.
Georg Andreas Sorge, Compendium Harmonicum (1760). Source: The Internet Archive. Digital facsimile from The University of North Carolina.
Gottfried Weber, Versuch einer geordneten Theorie der Tonsetzkunst (1817-21).
