For a while, last night in the Library of Congress, it seemed obvious that Ravel's Trio in A Minor was the greatest music ever composed for the combination of violin, piano and cello. The primary credit for this impression belongs to the composer, of course, but intimations of greatness seem to creep into most music when it is being performed by the Beaux Arts Trio. After the intermission, there was a similar depth and vitality in the Brahms Trio in C Minor, Op. 101, and there were even hints of greatness (particularly in the slow movement) in the Trio in G by Johann Nepomuk Hummel, which opened the program.

But the Hummel trio, composed by the only pupil of Mozart who achieved any lasting reputation, is a pleasant, unpretentious work, mingling traces of Mozartean style with a gentle, graceful romanticism. The Beaux Arts Trio reserved its most thundering climaxes and wild lyric episodes for the Brahms and Ravel works, where they belong, and presented Hummel on a smaller scale, beautifully proportioned and transparent in texture.

Beginning their second season in residence at the library, under the auspices of the William and Adeline Croft Fund, pianist Menahem Pressler, violinist Isidore Cohen and cellist Bernard Greenhouse were in rare form last night, even by the lofty standards they have established. They played with a total unanimity of phrasing, dynamics and emotional interpretation, a deep grasp of structures and a spontaneity that can come only through years of playing together. By now, the trio has reached total, intuitive agreement on the smallest points of interpretation, almost as though a single player were performing simultaneously on piano, violin and cello.

The Ravel received a virtuoso performance, light and nimble in the second movement and almost orchestral in the power of its climaxes. It was particularly impressive in its subtly shifting textures and balances, in the sense of continuity, in the rich emotional flavor achieved at slow tempos.

The Brahms is stormy music, lyrical, graceful and occasionally ponderous. It often requires virtuoso technique, but it presents, on the whole, a more straightforward set of challenges, which the Beaux Arts Trio met with precision and obvious enjoyment. This year's three concerts by the trio will include all three of the Brahms works in this form, marking the sesquicentennial of his birth. Last night's experience makes that a very pleasing prospect. The concert will be repeated tonight at 8, and broadcast live on WETA-FM.