The precise psychophysical mechanism through which the different moods are aroused is for the most part hidden from us; yet in certain particulars we can form some idea of it. For example, the richness of feeling in the tones of certain instruments as compared with others is doubtless due to the fact that through the presence of more overtones and the admixture of noise, the reaction is more complex; the tense excitement of high and loud tones, as compared with the soft and low, is probably connected with the fact that their higher vibration rate and greater amplitude of vibration produce a more marked effect, a more pervasive disturbance,--the organism does not right itself and recover so rapidly and easily. These direct and native elements of feeling are then broadened out and intensified through other elements that come in by way of association. For example, in order to sing high tones, a greater tension and exertion of the vocal chords is needed than for low tones; loud tones suggest loud noises, which, as in breaking and crashing and thundering, are inevitably associated with fear; the loud is also the near and present and threatening, the low is distant and safe. Although each tone, as separate and individual, possesses its own feeling in its own right, the tonal effects are immensely accentuated by contrast with one another,--the high against the low, the poor against the rich, the loud against the soft--and through the summation, by means of repetition, of the influences of many tones of like character; the full meaning of music depends upon the relations of tones, especially the temporal relations.