Have we not here, then, adequate data for a theory of music? These vocal peculiarities which indicate excited feeling, _are those which especially distinguish song from ordinary speech_. Every one of the alterations of voice which we have found to be a physiological result of pain or pleasure, _is carried to its greatest extreme in vocal music_. For instance, we saw that, in virtue of the general relation between mental and muscular excitement, one characteristic of passionate utterance is _loudness_. Well, its comparative loudness is one of the distinctive marks of song as contrasted with the speech of daily life; and further, the _forte_ passages of an air are those intended to represent the climax of its emotion. We next saw that the tones in which emotion expresses itself, are, in conformity with this same law, of a more sonorous _timbre_ than those of calm conversation. Here, too, song displays a still higher degree of the peculiarity; for the singing tone is the most resonant we can make. Again, it was shown that, from a like cause, mental excitement vents itself in the higher and lower notes of the register; using the middle notes but seldom. And it scarcely needs saying that vocal music is still more distinguished by its comparative neglect of the notes in which we talk, and its habitual use of those above or below them and, moreover, that its most passionate effects are commonly produced at the two extremities of its scale, but especially the upper one. A yet further trait of strong feeling, similarly accounted for, was the employment of larger intervals than are employed in common converse. This trait, also, every ballad and _aria_ carries to an extent beyond that heard in the spontaneous utterances of emotion: add to which, that the direction of these intervals, which, as diverging from or converging towards the medium tones, we found to be physiologically expressive of increasing or decreasing emotion, may be observed to have in music like meanings. Once more, it was pointed out that not only extreme but also rapid variations of pitch, are characteristic of mental excitement; and once more we see in the quick changes of every melody, that song carries the characteristic as far, if not farther. Thus, in respect alike of _loudness_, _timbre_, _pitch_, _intervals_, and _rate of variation_, song employs and exaggerates the natural language of the emotions;--it arises from a systematic combination of those vocal peculiarities which are the physiological effects of acute pleasure and pain.