Apart from all differences of opinion or analogies with other works, it may safely be asserted that these quartets are the clear and perfect expression of Mozart's nature; nothing less is to be expected from a work upon which he put forth all his powers in order to accomplish something that would redound to his master Haydn's honour as well as his own. The form had already, in all its essential points, been determined by Haydn; it is the sonata form, already described, with the addition of the minuet--in this application a creation of Haydn's. Mozart appropriated these main {MOZART'S AND HAYDN'S QUARTETS.} (7) features, without feeling it incumbent on him even to alter them. Following a deeply rooted impulse of his nature, he renounced the light and fanciful style in which Haydn had treated them, seized upon their legitimate points, and gave a firmer and more delicate construction to the whole fabric. To say of Mozart's quartets in their general features that, in comparison with Haydn's, they are of deeper and fuller expression, more refined beauty, and broader conception of form,[17] is only to distinguish these as Mozart's individual characteristics, in contrast with Haydn's inexhaustible fund of original and humorous productive power. Any summary comparison of the two masters must result in undue depreciation of one or the other, for nothing but a detailed examination would do full justice to them both and explain their admiration of each other. Two circumstances must not be left out of account. Mozart's quartets are few in number compared with the long list of Haydn's. Every point that is of interest in Mozart may be paralleled in Haydn; hence it follows that certain peculiarities found in Haydn's music are predominating elements in Mozart's. Again, Haydn was a much older man, and is therefore usually regarded as Mozart's predecessor; but the compositions on which his fame chiefly rests belong for the most part to the period of Mozart's activity in Vienna, and were not without important influence on the latter. This mutual reaction, so generously acknowledged by both musicians, must be taken into account in forming a judgment upon them. The string quartet offers the most favourable conditions for the development of instrumental music, both as to expression and technical construction, giving free play to the composer in every direction, provided only that he keep within the limits imposed by the nature of his art. Each of the four combined instruments is capable of the greatest variety of melodic construction; they have the advantage over the piano in their power of sustaining the vibrations of the notes, so as to produce song-like effects; nor are they inferior {MOZART'S INSTRUMENTAL MUSIC.} (8)