The column that he wrote for 37 years was imposing, and impossible, and so is Joseph Alsop. He is erudite, invulnerable, privileged, outrageous. He is less an inkstained wretch than a Washington grandee. If you wonder how Joe Alsop came to be invited to deliver next year's Andrew W. Mellon Lectures in the Fine Arts at the National Gallery of Art, you might consider this: He once was very fat.

Now, at 67, slim and retired from the papers, Joseph Alsop is still cushioned, and partially concealed. Layers of high luxury, of learning and of self-indulgence, separate him from others in the world.

Alsop, the reporter, also held us at a distance. Whether issuing his warnings of the peril posed by the Soviets, or denouncing Joe McCarthy, or defending, to the end, our war in Vietnam, Alsop never stroked his readers. He addressed us from afar.

He found reporting easy- his intellect is formidable, his memory amazing, and he wrote with speed - but if covering the news did not test him fully, at least it held its pleasures. Alsop is a hungry man who savors as he feeds. Alsop, the reporter, insatiably ingested anecdotes and quotes and what he called The Facts.

He did not tell us all of them. For even as Joe Alsop chatted with the Presidents, offered them advice, listened to their secrets and blasted them in print, part of his attention was focused on his "other life."

"I never talked to anyone about it," Alsop says.

For 14 years, and longer, he has been privately covering a story that has not yet appeared. It is not about the Senate or the KGB. When he rises here next June to inaugurate the lecture hall of the National Gallery's East Building, Alsop will instead report on Asia Minor, China, the history of art history and of art collecting.

"How did my two lives mix? My dear, they did not mix at all."

Alsop, when at home in his rented house in Georgetown, lives in the grand manner, surrounds himself with objects d'art, sits on down-filler pollows. "When Joe is good," says one old friend, "he's very, very good, and when he's bad he's horrid." Alsop, now the gracious host, is very good indeed.

Sun pours through tall windows,' gleams on polished wood, glints on antique silver. The cook is in the kitchen, the gardener in the garden, lunch will soon be served.

Joseph Alsop's bar stands before his bookcase, and he must turn his back, if only for a moment, to pur the first round of the day. Just before he does, Alsop asks a question, part pleasantry, part ploy.

"What might you do, dear boy, that would make you feel immoral?"

His accent is High Troy and in it one hears Groton, the great houses of Connecticut, old money, private clubs. He speaks with such precision that one is expected to respond with care. Immoral? Before one forms, or finds, what seems a proper answer, a proper answer, a drink is in one's hand.

Alsop answers his own question:

"I would feel immoral were I to purchase caviar."

Alsop's Yankee ancestors-he dines among their portraits-bequeathed him more than money. In Dictionary of American Biography we read that Richard Alsop (1761-1815), the satirist and poet, "was one of the few millionaires of his generations, and was also, in many ways, the most gifted of the Hartford Wits." (He read French, Spanish, Italian, and the Scandinavian languages, and wrote poetry in Creek.)

Joseph Wright Alsop Jr. (Groton '28, Harvard '32) was born Oct. 11, 1910 in Avon, Conn. "These roots of mine are peculair by any current standard" he wrote in 1974. "The little world of the Farmington River Valley, where I grew up, was much admired by Henry James on his rather unhappy return trip to his native land . . . Except that my family and their friends in the valley were rather more comfortable than they probably deserved, there was only one small feature of the small world that still puzzles and strikes me. These friends and relations-for there were a lot of relations-were most uncommonly preoccupied in with history as a process."

So is Alsop. His ancestors were bookish, and he loves to read; his ancestors were witty, and his conversation darts, surprises. His ancestors were cunning merchants. What is wrong with caviar is that it is not a good investment. If Alsop loves his luxuries, he loves a bargain, too.

"Retirement to be blunt and practical about it, always requires a man to cash in his savings. My savings, somewhat eccentrically, have gone into a much-loved house and garden and the house's endlessly pleasurable contents," wrote Alsop in a farewell column about the time he sold his house and cashed in his things.

Many yet remain. Standing now among them, Alsop speaks approvingly of the 2,220-year-old figurines he bought years ago in China, mentioning their rarity, their style, their antiquity. He does not cite their beauty. "They cost me 25 cents a piece."

His handmade shirts are monogrammed, their cuffs are linked with heavy gold. Hif suits are made in Saville Row. at a price that he deems reasonable. His shoes were made in London too, until he turned 60. "When they began to ask $300 a pair, I turned to Hong Kong."

When Alsop speaks of art, he rarely mentions taste, but often mentions cash. "I was younger than you are dear boy, when I made up my mind to buy 10 pictures by Paul Klee. Ten for $2,500. I suggested to my father I simply had to have them. He said, "If you do I shall never speak to you again." I have since found five or six of them in Nelson Rockefeller's front hall."

By now lunch is ready. "I have never collected art," he says."I never had the money."

Alsop for an hour, has been talking about art, scholars and ideas. "You can't suppose this sort of chat is popular at Groton," he says. "The people I liked most when I was young were rather dashing people. Dashing people, except perhaps in England, have no esthetic interest. If you talk about art, or a book, at even the finest dinner in Washington, everybody goes sound asleep, covered with cobwebs, until the frog arrives to kiss the princess and the subject drops."

Alsop. at the Gallery, will lecture not on taste, but on "Art Collecting as a Phenomenon in Art History."

The fresh pasta with fresh spinach has been cleared away. The cook appears to pour the second wine and to pass a platter of Virginia ham. "It took me 30 years to find a fine Virginia ham," says Alsop. "What art collecting is has never been defined.

"It is not merely patronage, patronage is common. It is not treasure gathering, man has the magpie instinct, and treasure gathering is universal. True collecting arrives when people respond to art historically, rather than esthetically. You have an enormous variety of human cultures, many of which have produced very great art. Yet only a very few have produced collectors and collecting-a peculiar behavioral system of extreme complexity. I have been trying to see why."

Alsop is by now warming to his subject, and he tells a story well. He speaks of Attalus of Pergamum, of Lysamichus ("a brilliant general without luck"), and of his eunuch treasurer. He talks of Egypt, Crete and China, and of the Gauls, defeated, who settling in Asia Minor, became St. Paul's Galatians.

Alsop had been speaking of Imhotep, the Egyptian, who may have been the first man to sign a work of art, when suddenly he paused.

"Dear boy," he said. "Please forgive me. I must ask a favor. Tonight I am expected at a seated dinner. Old friends of mine will be there.I have, you know, been ill. I am unable to attend. Might you go in my place?"

Should you wish to understand Joseph Alsop's Washington, try, for just an evening, to step into his world as his wan replacement, his Designated Eater.

The party, as such parties do, begins at 8:15 and ends, as if by order, at 11:30 promptly. The tables have been set in the barrel-vaulted glass-roofted hall that overlooks the swimming pool. The hostesses this evening are Kay Evans of Georgetown and Susan Mary Alsop, the dear friend of Joseph Alsop's who is his former wife. The occasion is a rare visit to this city by Nigel Nicolson of Britain, a son of famous parents who wrote about them in his "Portrait of a Marriage." Kay Evans' husband, Rowland, greets you at his door. You have worked for the same paper for a dozen years, but you move in different circles and have never met.

The guests include the David Brankleys, Sen. Abraham Ribicoff, the wife of the British ambassador, and from the State Department, Hodding Carter 111. Their faces are familiar, but they do not know yours. Susan Mary Alsop, a woman of great warmth, notices your isolation and takes you by the hand.

"I hear," she says, "you are writing on Joe's other life a sort of Alsop's Prufrock. There is a story that might help. Perhaps he told you. He was just a little by, seven Years old, or eight. It was Christmas or his birthday. His mother asked him what he wanted. She was astonished by his answer. He said he wanted to go to New York to the Metropolitan Museum of Art to see the artifacts from Crete. It seems that he'd been reading the Encyclopaedia Britannica."

The meal is delicious. Joseph Alsop, it turns out, has lent the house his cook.

"I have known Joe Alsop well for more than 40 years," says Ambassador David K.E. Bruce. "His memory is awesome. I asked him a few modays ago about the suicide of Cicero. Had he taken hemlock? Had he fallen on his sword? Of course Joe Knew the answer, and a string of anecdotes curious and amusing.

"Joe Alsop is an exceptionally learned man," the ambassador continued. "He is occasionally abrasive. But there is another side to Joe which you may not have been. I spoke sharply with him long ago on a matter purely personal. Since then, for 40 years, Joe has exercised great tact."

"There is nothing that Joe Alsop admires more than courage, moral courage," said Evangeline Bruce, the ambassador's wife. "And he is himself courageous. He did not want to be disliked, yet he reportedly bravely. He would sometimes lose his friends, which he minded very, very much, but he would not bend. I sometimes think that Joe was toughened as a young man. At the age when one feels the need to be particularly attractive to others, he wasn't. He was brainy, he was plump."

In 1937, the young Joseph Alsop, the 250-pound reporter, waged war on his fat. He spent three months reducing in the Johns Hopkins Hospital in Baltimore, and shed 80 pounds. The bill was not a small one. Alsop paid it with the proceeds of "How it feels to Look Like Everybody Else," an article he sold to the Saturday Evening Post.

Alsop was imprisoned by the Japanese. He served in China in the war as an aide to Maj. Gen. Claire Chennault of the Flying Tigers. He landed with the Marines at Inchon in Korea, and saw combat yet again in the paddies of Vietnam. He wrote that battle rather thrilled him.

"I feel two ways about the Mellon Lectures," Alsop said. "It is a very great honor, it is the most honorable lectureship there is, but to leap from writing on Vietnam to the National Gallery of Art is not easy. I know I've done my homework, I know I know my books. Still, it scares the life out of me."

Alsop has a theory. He has been honing and refining it for the past 14 years. It deals with a system, a system that explains "the history and character of the phenomena of art namely art collecting, art history and the art market, plus such secondary phenomena as art museums. You find that this system has only developed in six art traditions, and has therefore been unknown during most of the history of art on earth. That fact alone raises a whole series of previously unsuspected but important questions which have been asked, yet badly need to be answered."

Why are there art collectors? Why do they feel they have to have a Raphael, a Rembrandt? When did collectors learn to value works of art not for their beauty or their cost, but for the process that they manifest? Why did the Minoans develop a great liking for the hard stodne vessels of the early Egyptians? "What leads art collectors to reevaluate the past, to like African sculpture better than the Apollo Belvedere? Why do they buy antiques, of which this poor old house is full? Why do they pay super prices for old works of art? How people saw thing in China, or in Asia Minor, or in Venice in the Renaissance, is something you have to deduce. How people saw things is not irrelevant to the history of art."

Joseph Alsop knows he has bitten off a lot. When he speaks of politicians, his tone is often cutting. When he mentions the art historians whom he most admires-England's Sir Ernst Gombrich or Professor Meyer Schapiro, the legendary scholar who has taught for more than 50 years at Columbia College - he uses terms of awe.

"I view Meyer Schapiro," says Alsop, "as an unapproachable saint. To speak with Schapiro-it really is dazzling, it's beyond conception. Meyer Schapiro is the most learned man I know."

They are a touchy breed, the august art historians. Next year's Mellon Lecturer will be scrutinized most pointedly, for he is, as they well know, not a polished scholar. He is a retired journalist without advanced degrees - an amateur.

Alsop once held enormous power. He was among the rulers of the ruling class. Franklin Delano Roosevelt was a cousin. John Fitzgerald Kennedy, on the snowy night of his Inaugural, paid a call of courtesy to Joe Alsop's Georgetown house. But the papers come out every day, and the prestige of the reporter does not linger long. Alsop's once was great and he made the grandees tremble; but it has been much diminished by his writings on Vietnam. Many feel that his day has come and gone.

At the end of 1974 - the year that he retired, and the year his brother and onetime partner, Stewart, died - Joseph Alsop wrote that at that moment "I felt a final failure of all the zest and gusto and eagerness to know what will happen next that are the impelling motives of any good reporter . . . Oddly, he who was doomed to die felt no loss of zest at all. He diagnosed my feelings on the main grounds. 'You belong to the past much more than I do,' he said.I am now sure that he was right."

The afternoon was ending. "I often wish," said Joseph Alsop, "that I'd become a scholar."

He was standing on his doorstep; the autumn sun was setting. "Think of the societies flattened. History has a rule: Nothing endures," he said. "It is very nice to endure."