instrumental polyphonic works which have never been equalled and which are still the fountainheads of our musical learning. It ought to be noted that his own perfection of the system of equal temperament enabled him to do much that his predecessors could not have done, even had they possessed his genius. By making it possible to play in all twenty-four keys, and to modulate from the tonic of a composition into very remote keys, Bach introduced into instrumental polyphony an elasticity, a pliancy, a freedom, which it had never before possessed. He was able to fill his polyphonic writing with the passionate utterance of chromatic harmony; and in his "Chromatic Fantasia and Fugue" he produced a work which was actually a bridge between the style that went before him and the style that followed him. With Bach the development of instrumental polyphony came to an end. Nothing has been added to its technic except the application of the most recent laws of harmony. Bach was ahead of his time, and music was working out simpler problems than his when his work was completed. They were problems in monophonic style, and to these we must now turn our attention. Chapter X Monophonic Style and the Sonata Corelli and his violin style--C. P. E. Bach and his departures from polyphony--General plan and purpose of the early sonata--Haydn and his two principal themes--Mozart and song-melody--Clementi and the influence of his style--Beethoven's improvements in sonata form--His employment of instrumental music for emotional expression.