AIR, in Music, is taken in different senses. It is sometimes contrasted with harmony, and in this sense it is synonymous with melody in general.—Its proper meaning is, a tune, which is set to words, or to short pieces of poetry, that are called songs.
In operas we give the name of air to such pieces of music as are formed with measures and cadences to distinguish it from the recitative; and, in general, every piece of music is called an air which is formed for the voice, or even for instruments, and adapted to stanzas, whether it forms a whole in itself, or whether it can be detached from any whole of which it forms a part, and be executed alone.
If the subject admits of harmony, and is set in parts, the air is, according to their number, denominated a duet, a trio, a quartetto, &c. We need not follow Rousseau and the other philologists in their endeavours to investigate the etymon of the word air. Its derivation, though found and ascertained, would contribute little to illustrate its meaning in that remote sense to which, through a long continuance of time, and the various vicissitudes of language, it has now passed. The curious may consult the same article in the Dictionnaire de Musique by M. Rousseau.
In modern music, there are several different kinds of airs, each of which agrees to a certain kind of dancing; and from these dances the airs themselves take their specific names.
The words to which airs are adapted are not always rehearsed in regular succession, nor spoken in the same manner with those of the recitative; and though in general they are very short, yet they are interrupted, repeated, transposed, at the pleasure of the artist. They do not constitute a narrative, which once told is over; they either delineate a picture which it is necessary to contemplate in different points of view, or inspire a sentiment in which the heart acquiesces with pleasure, and from which it is neither able nor willing to be disengaged; and the different phrases of the air are nothing else than different manners of beholding the same image. This is the reason why the subject of an air should be one. It is by these repetitions properly placed, it is by these redoubled efforts, that an impression, which at first was not able to move you, at length shakes your soul, agitates you, transports you out of yourself; and it is likewise upon the same principle that the runnings, as they are called, or those long, mazy, and inarticulate inflections of the voice, in pathetic airs, frequently seem, though they are not always so, improperly placed; for whilst the heart is affected with a sentiment exquisitely moving, it often expresses its emotions by inarticulate sounds, more strongly and sensibly than it could do by words themselves.
The form of airs is of two kinds. The small airs are often composed of two strains, which ought each of them to be sung twice; but the important airs in operas are frequently in the form of rondeaus.