When composers grow verysk,1 sw,-1 ld,10 old and continue to write, the results sometimes take on a very special eloquence. There are no formalities for the sake of formalities, and lyric expression tends to be pure and concise. Consider Verdi, in "Falstaff"; Strauss, in the "Four Last Songs"; and, from last night's fine program by the Beaux Arts Trio, the deeply moving piano Trio in D minor that Gabriel Faure' wrote at the age of 78.

Much of Faure''s music is serene (the warmly comforting Requiem being the most famous example), but in this beautiful trio there is also a mellow valedictory dimension. The broad, rich opening theme deep in the cello envelops like a deep embrace, only to be answered in a similar mode by the violin. This slow, unruffled movement is almost a duet for cello and violin, with the piano providing a rich foundation. Faure''s acute harmonic and tonal sensibilities produce themes of real beauty and subtlety.

Violinist Isidore Cohen and cellist Bernard Greenhouse played this music, which is all too little known, with great feeling.

If the opening movement is slow, the following one is even slower, with special touches of harmonic ambivalence that are the only slight sign that this music came from 1922-'23, a decade after the premiere of "Le Sacre du Printemps" and five years following the death of Faure''s stylistic successor, Debussy. The finale sustains the valedictory mode.

The concert, which was part of the trio's annual series at the Library of Congress and will be repeated tonight, opened with a late Haydn trio, the one in D major, H. XV:25. It is also fairly serene, at least for the usually vital, witty Haydn. The first movement is a slow one, a short set of variations on a delicate, graceful theme. It is followed by a broad adagio, with some grave diversions into minor keys. The finale is more like the Haydn we expect, a "Rondo all'Ongarese" with some jaunty ventures into Hungarian themes. The Beaux Arts played the trio with delectable deftness.

Dvora'k's Trio in F-minor, Op. 65, a four-movement work, concluded the program. Only the second of the movements seemed to be really top drawer Dvora'k, with its Bohemian mood, unified as it is with consistent cross rhythms and evanescent textures. The rest is certainly attractive, but on the whole lacks the irresistible lure of the composer at his most passionate. The music tends to be a little diffuse. It was beautifully played, especially by Menahem Pressler in the demanding piano part.

The encore was that wonderfully madcap romp with which Beethoven concluded his Trio, Op. 1, No.2. One cannot hear this work without thinking that it must have been Rossini's inspiration for the end of the "William Tell" Overture. What a delightful piece.