Beethoven's C Major Quartet is the last and the least profound of his great Opus 59. It is also a work of sheer joy, and all of its simple, ineffable happiness, came alive in the hands of the Muir String Quartet last night at the Library of Congress.

An air of adventure hung on the short introduction, as the modulations revealed the main theme. The flexible tempi allowed the music to breathe freely and the playful 6/8 andante seemed to glow in the light of the prevailing key of A minor. Not content with grace and glee, the Muir also explored the curious darkness that lurks in the third movement. The brief minuet, perhaps the least danceable such movement in the literature, was properly Teutonic, as was the laughter of the final allegro. Then the fugal finale simply happened. Beethoven wrote the fugue as if in one breath, and so the Muir Quartet played it, with seamless line and thrilling vigor. It was a fabulous performance.

Debussy's Quartet in G minor, Op. 10, preceded the Beethoven. The myriad colors of this score all flow from a single rainbow, and its monothematic insistence must be made persuasive in practice. Last night the emotional and dynamic range of the music was not broad enough, although there were several nice touches: the third movement, while falling apart rhythmically, offered glorious legato; and the pizzicati in the second movement were lively and crisp.

The evening began with Haydn's Quartet in B minor, Op. 33 No. 1. There was strength but not much elegance, the rests in the first allegro wanted patience and the intonation was curiously shy. The Muir's matching Stradivari need matching sensibilities for the proper blend. The silken textures of the Debussy and Beethoven readings showed that these gifts were there, but they had not arrived yet for Haydn. The concert, which was broadcast live on WETA, will be repeated in part on National Public Radio's Sunday Show.