Though we sometimes meet with Operatic Airs of the 18th century which seem, at first sight, inconsistent with this rigid system of classification, a little careful scrutiny will generally enable us to refer them, with tolerable certainty, to one or other of the universally-recognised orders.
The Cavatina, for instance, distinguished from all other types by the absence of a Second Part and its attendant Da capo, is, in reality, nothing more than an abbreviated form, either of the Aria cantabile, the Aria di portamento, or the Aria di mezzo carattere, as the case may be. The Second Act of 'Teseo' opens with an example which establishes this fact very clearly, needing only the addition of a subordinate Strain in order to convert it into a regular Aria cantabile.
The Aria d'imitazione was written in too many varieties of style to admit the possibility of its restriction to any single Class. Warlike Airs with Trumpet obbligato, Hunting-Songs with Horn Accompaniment, Echo-Songs—such as 'Dite che fà,' in 'Tolomeo'—Airs with obbligato Flute passages or vocal trills suggestive of the warblings of birds, and descriptive pieces of a hundred other kinds, all fell within this category, and generally exhibited the prominent characteristics of the Aria di mezzo carattere, unless, as was sometimes the case, they were simple enough to be classed as Arie cantabili, or even Arie parlanti, with a more or less elaborate obbligato Accompaniment, or contained volate of sufficient brilliancy to enable them to rank as Arie d'agilità.
The Aria all' unisono is of comparatively rare occurrence. 'Bel piacer,' sung by Isabella Girardeau, in 'Rinaldo,' and generally regarded as the typical example of the style, is a pure Aria cantabile, written for an expressive Soprano, supported only by a single Violin part, playing in unison with the Voice throughout. In the Symphonies, a Violoncello part is added; but it is never heard simultaneously with the Singer. Similar Airs will be found in 'Il Pastor Fido' and 'Ariadne'; but we meet with them so seldom, that it is doubtful whether they were ever held in any great degree of favour, either by Singers or the public. The fine Song, 'Il tricerbero umiliato,' in 'Rinaldo,' represents a less rare form, wherein the Basses and other Instruments all supported the Voice in Unisons or Octaves.
The Aria concertata was simply an Aria di mezzo carattere, or an Aria parlante, with a more than usually elaborate or original Accompaniment. Among the finest-known examples of this class, we may mention 'Priva son,' in 'Giulio Cesare,' with Flute obbligato; 'Hor la tromba,' in 'Rinaldo,' with four Trumpets and Drums obbligati; an Air in 'Il Pastor Fido,' with Accompaniments for Violins, and Violoncellos in Octaves pizzicato, with a Harpsichord part, arpeggiando, throughout; 'Ma quai notte,' in 'Partenope,' accompanied by a Flutes, 2 Violins, Viola, and Theorbo, with Violoncelli and Bassi pizzicato; 'Se la mia vita,' in Ezio,' for 1 Violin, Viola, Violoncello, 2 Flutes, and 2 Horns; 'Alle sfere della gloria,' in 'Sosarme,' for the Full Stringed Band, enriched by 2 Oboes, and 2 Horns; and a highly characteristic Scena, in 'Semele'—'Somnus, awake!' for 2 Violins, Viola, Violoncello, 3 Bassoons, and Organ.
The sequence and distribution of these varied Movements was regulated by laws no less stringent than those which governed their division into separate Classes. It was necessary that every Scene in every Opera should terminate with an Air; and every member of the Dramatis personæ was expected to sing one, at least, in each of the three Acts into which the piece was almost invariably divided; but no Performer was permitted to sing two Airs in succession, nor were two Airs of the same Class allowed to follow each other, even though assigned to two different Singers. The most important Airs were played at the conclusion of the first and second Acts. In the second and third Acts, the hero and heroine each claimed a grand Scena, consisting of an Accompanied Recitative—such as 'Alma del gran Pompeo,' in 'Giulio Cesare'—followed by an Aria d'agilita calculated to display the power of the Vocalist to the greatest possible advantage; in addition to which the same two characters united their Voices in at least one grand Duet. The third Act terminated with a Chorus of lively character, frequently accompanied by a Dance: but no Trios, Quartets, or other Concerted Movements were permitted in any part of the Opera, though three or more Characters were sometimes suffered—as in 'Rinaldo'—to join in a harmonised exclamation, at the close of a Recitative.
- More than seventy years afterwards, Mozart used the same expedient, with Irresistible effect. In 'Le Nozze di Figaro.' Old Opera-goers will scarcely need to be reminded of the frantic 'double encore' which followed the delivery of the words, 'E schiatti il Signer Conte al gusto mio.' by Mlle. Jenny Lind, Mme. Grimaldi, Signor Lablache and Herr Staudigl, at Her Majesty's Theatre, in the year 1847.