=The Various Qualities of Sound.=--Physically, sounds consist of vibrations, and these are, generally speaking, _aërial waves_. When the waves are non-periodic the result is a _noise_; when periodic it is what is nowadays called a _tone_, or _note_. The _loudness_ of a sound depends on the _force_ of the waves. When they recur periodically a peculiar quality called _pitch_ is the effect of their _frequency_. In addition to loudness and pitch tones have each their _voice_ or _timbre_, which may differ widely in different instruments giving equally loud tones of the same pitch. This voice depends on the _form_ of the aërial wave. =Pitch.=--A single puff of air, set in motion by no matter what cause, will give a sensation of sound, but it takes at least four or five puffs, or more, to convey a sensation of pitch. The pitch of the note _c_, for instance, is due to 132 vibrations a second, that of its octave _c´_ is produced by twice as many, or 264 vibrations; but in neither case is it necessary for the vibrations to go on during a full second for the pitch to be discerned. "Sound vibrations may be too rapid or too slow in succession to produce sonorous sensations, just as the ultra-violet and ultra-red rays of the solar spectrum fail to excite the retina. The highest-pitched audible note answers to about 38,016 vibrations in a second, but it differs in individuals; many persons cannot hear the cry of a bat nor the chirp of a cricket, which lie near this upper audible limit. On the other hand, sounds of vibrational rate about 40 per second are not well heard, and a little below this they produce rather a 'hum' than a true tone-sensation, and are only used along with notes of higher octaves to which they give a character of greater depth."[22] The entire system of pitches forms _a continuum of one dimension_; that is to say, you can pass from one pitch to another only by one set of intermediaries, instead of by more than one, as in the case of colors. (See p. 41.) The whole series of pitches is embraced in and between the terms of what is called the musical scale. The adoption of certain arbitrary points in this scale as 'notes' has an explanation partly historic and partly æsthetic, but too complex for exposition here.