Prominent among the materials welded into it was the dance of obscure origin. As the vocal aria was the result of the simple folk-song combined with the intense craving of song's master molders for individual expression, so instrumental music striving to walk alone, without support from words, gained vital elements through the discovery that various phases of mental disposition might be indicated by alternating dance tunes differing in rhythm and movement, according to Nature's own law of contrasts. That unity of purpose was essential to the effectiveness of the diversity was instinctively discerned. The touch of authority was given to this kind of music, during the last two decades of the seventeenth century, by Arcangelo Corelli when he presented in the camera, or private apartment, of Cardinal Ottoboni's palace, in Rome, his idealized dance groups, thoroughly united by harmony of mood, yet affording a wholly new tone-picture of this mood in each of several movements. These compositions were usually written for the harpsichord and perhaps three instruments of the viol order, the master himself playing the leading melody on the violin. He called them sonatas from sonare, to sound, a name originally applied to any piece that was sounded by instruments, not sung by the human voice. They prefigured the solo sonata, the entire class of chamber music named from the place where they were performed, and the symphony which is a sonata for the orchestra. Absolute music was set once for all on the right path by them. They ushered in a new era of Art. Purcell, in England, Domenico Scarlatti and Sammartini, in Italy, the Bachs, in Germany, and others continued to fashion the sonata form. It ceased to be a mere grouping of dances, the name suite being applied to that, and struck out into independent excursions in the domain of fancy. The prevailing melody of its monophonic style proved suitable to furnish a subject for the most animated discussion. Three contrasting movements were adopted, comprising a summons to attention, an appeal to both intellect and emotions, and a lively reaction after excitement. A German critic has jocosely remarked that the early writers meant the sonata to show first what they could do, second what they could feel, and third how glad they were to have finished. Time vastly increased its importance. Two subjects, a melody in the tonic, another usually in the dominant, came to set forth the exposition of the opening movement, leading to a free development, with various episodes, and an assured return to the original statement. The prevailing character being thus defined, the story readily unfolds, aided by related keys, in a slow movement and perhaps a minuet or scherzo, and gains its denouement in a stirring finale, written in the original key. Each movement has its own subjects, its individual development, with harmony of plan and idea for a bond of union.