To meet this demand music as a science has evolved a tone system. That is, from the infinite number of tones it has selected something over a hundred having definite mathematical relationships, fixed vibrational ratios. The art of music takes this system of tones and by means of combinations, progressions and movements which constitute what is called musical composition, it undertakes to excite a wide variety of emotions.
The aim and office of music is to create moods. It does not arrive at definite expression. There is no musical progression which is universally understood as an invitation to one's neighbor to pass the bread. The pianist cannot by any particular tone combination make his audience understand that his left shoe pinches, but he can make them smile or look serious. He can fill them with courage or bring them to tears without saying a word. In listening to the Bach _B Minor Mass_ one can tell the _Sanctus_ from the _Gloria in Excelsis_ without knowing a word of Latin. The music conveys the mood unmistakably.
A song is a union of music and poetry, a wedding if you please and as in all matrimonial alliances the two contracting parties should be in harmony. The poem creates a mood not alone by what it expresses directly but by what it implies, what it suggests. Its office is to stimulate the imagination rather than to inform by direct statement of facts. The office of music is to strengthen, accentuate, and supplement the mood of the poem, to translate the poem into music. The best song then, will be one in which both words and music most perfectly create the same mood.
Arnold Bennett's definition of literature applies equally well to the song. He says: "That evening when you went for a walk with your faithful friend, the friend from whom you hid nothing--or almost nothing--you were, in truth, somewhat inclined to hide from him the particular matter which monopolized your mind that evening, but somehow you contrived to get on to it, drawn by an overpowering fascination. And as your faithful friend was sympathetic and discreet, and flattered you by a respectful curiosity, you proceeded further and further into the said matter, growing more and more confidential, until at last you cried out in a terrific whisper: 'My boy she is simply miraculous:' At that moment you were in the domain of literature." Now when such impassioned, spontaneous utterance is brought under the operation of musical law we have a perfect song. The composer furnished the words and music, but the thing which makes it a song comes from the singer, from the earnestness and conviction with which he delivers the message.