Early in this century, while nearly all major American collectors were still fuming over or laughing at modern art, a tall and randy New York lawyer named John Quinn was keeping the avant-garde afloat almost single-handedly with his purchases.

Between 1911 and his death in 1924 Quinn bought or commissioned about 2,000 works by, among many others, Brancusi, George Russell, Duchamp-Villon, Matisse, Augustus and Gwen John, Picasso, Cezanne, Van Gogh, Gauguin, Jack and John Butler Yeats, Braque, Prendergast, Dufy, Rivera, Rousseau and Roualt.

While he bought many outstanding works for a song, he also bought some dogs, and tended to slight American artists, deserving or otherwise. He listened to the critics and spoke deprecatingly of his uncultured back-ground and taste, but chose for himself.

Had he never bought so much as a doodle Quinn would have been a significant contributor to modern art, because he was one of the principals behind the famous (or, to most of the critics of the day, notorious) 1913 Armory Show, which established the American beachhead of the modernists.

Quinn's magnificent collection, which filled his 11-room Manhattan apartment to bursting, was sold at auction, as directed by his will. It was an oddly callous final act by a man who had been perhaps the most percipient patron of those who led the assault on the conventions of art. Even at the time, the dispersal was mourned by many who recognized the Quinn collection as what could have been the core of a great museum.

Visitors to the Hirshhorn Museum this summer can experience a hint of what a Quinn Museum might have been. Seventy-nine works from the Quinn collection have been tracked down and borrowed by the Hirshhorn's Judith Zilczer. She has written a companion catalogue ($7.25 in paperback) that tells more thanwas perviously known, perhaps as much as it is posible to know, but less than one wants to know, about this country-born coporate lawyer who made a great deal of money in the carpeted corridors of Wall Street and spent most of it buying the works of artists despised by the cultural establishment.

Dr. Zilczer has been forthright enough to include, where it is known, how much Quinn paid for each work. Even if you don't like modern art, it is titillating to know that Quinn was buying Picassos at four for $5,000, which was like buying Polaroid for $5 a share when it was called Haloid.

Quinn's dress and his pose were as severe and wooden as Woodrow Wilson's, but after office hours he hobnobbed with Irish revolutuinaries (he supported them until they got smartass with him), cerebrated with such writer as James Joyce, Ezra Pound and T.S. Eliot, and kept liaisons with a number of remarkable women, one of whom managed the avant-garde feat of cohabiting openly with Quinn without alienating her husband.

Why this bachelor father of modern art amde no provision for his children - for fostering the strange and temperamental people who push back the frontiers of art is a creative process second only to theirs - remains unanswered.

Zilczer has traced some 500 of the works Quinn owned, but the rest are scatered through public and private collections around the world. The Hirshorn exhibition ends with a WANTED poster she hopes will lead to the discovery of some of them. It is a fitting coda.