A fair bit of blather has been written over the years about how a chamber ensemble -- particularly the string quartet -- is a fraternal democracy in comparison with an orchestra under the stern hand of a conductor.

There is a certain amount of truth to that, but the plain fact is that the first violinist of a quartet is the dominant figure, performing many of the jobs of a conductor, such as setting tempos and shaping the phrases. (And that is especially true when a first violinist is so forceful a figure as the Juilliard Quartet's Robert Mann.)

The young and splendid Emerson String Quartet, which played last night at the Renwick Gallery and will play again tonight and tomorrow, has taken a fascinating step in the direction of that fraternal ideal.

The two violinists alternate in the first seat. Last night in Schubert's plangently melodic and rich A-minor quartet the tone was set by first violinist Philip Setzer. It was a leisurely, understated reading. Melodies were smoothly and gently articulated. Attacks were soft. Accents were downplayed. There was more mellowness than force.

But when the group played the exhilarating Mendelssohn D-major quartet, Eugene Drucker was first violin. It was a restless, dynamic reading. Melodies were exuberant and extroverted. Drucker's phrasing had a hard edge, and on several occasions his playing shot off real sparks. Attacks were precise and decisive.

For one who had never heard the Emerson before, it is possible only to speculate about how much the change in first violinists accounted for the contrasting styles. But it is true that at times the Emerson sounded like different ensembles--the Juilliard in the Mendelssohn and the Guarneri in the Schubert. That, of course, is justifiedly flattering comparison in either case.

Between the two works, the National Symphony's superb first clarinetist, Loren Kitt, played Brahms' autumnal F-minor clarinet sonata. One really doubts that any clarinet player of Kitt's generation can outmatch him in fullness and evenness of tone, in breath control and in innate musicality. The fine pianist was Wu Han.