There was something about the string quintet medium that appealed specially to Mozart, and that something was evident in the Smithson String Quartet's concert last night in the Renwick Gallery. Three of his quintets made up the program (a foretaste of the Smithson's next recording project): K. 515 in C, K. 406/516b in C minor (adapted from a wind octet) and the brilliantly moody K. 593 in D. It was an evening that combined elegance with eloquence in typically Mozartean balance.

Mozart used a second viola, rather than the second cello favored by Schubert and Boccherini, and sometimes played it himself, explaining that he liked to be "in the middle of things." This addition to the standard string quartet gives the music a sense of solidity in its mid-range, enhances the harmonic possibilities and gives the composer a much larger array of opportunities for balances, divisions, shifting alliances and dialogues -- all deftly exploited by Mozart and played with warm tone and fine coordination by the Smithson and guest violist Lisa Rautenberg.

The performance was on period instruments with gut strings and the lower-pitched tuning common in the 18th century. While not essential for music of this vintage, it works well. It worked particularly well last night in the Grand Salon of the Renwick -- a 19th-century room but not terribly different from the 18th-century Viennese rooms in which much of Mozart's music was first played. The program will be repeated tonight and is well worth hearing.