Javier Perez de Cuellar is a glass of fresh mineral water in a business running to flat ginger ale. He is an engaging and articulate man, ready to accept the Argentine-British crisis as a test of the world body's capacity to serve world peace. "I am not interested in personal prestige," he says. "I would like to enhance the credibility of the U.N."

Perez de Cuellar is a child of the U.N., a 62-year-old diplomat and international lawyer who had been Peru's permanent representative and then a high functionary, lately undersecretary for political affairs, before winning election as secretary- general late last year.

The Falklands/Malvinas crisis--that evenhanded slash rubs off from a visit to the 38th floor--is the first occasion a broad public has had to focus on Perez de Cuellar, but he has been focusing on peacemaking for years.

To the public, he observes, the SG incarnates the U.N.--a role he accepts in good grace, although he does feel that Americans are unfair: they get mad at the U.N. when it departs from American policy, as in the Mideast, and ignore occasions when it does not.

To Perez de Cuellar, on the other hand, the SG is one of the United Nations' instruments of power, and the question is how to wield it. The charter endows the SG with only limited and vague powers, but the position has "tremendous moral powers." He cites President Franklin Roosevelt's conception that the SG should be an "international moderator--to moderate the positions of the parties in order to make agreement possible."

Among his predecessors he prefers Dag Hammarskjold, for "his capacity to put aside his problems and concentrate on translating French poetry into Swedish." Perez de Cuellar, too, escapes at the end of the day into music and books. Currently he is reading Henry de Montherland's "La Reine Morte."

He admires Hammarskjold as well for "the way he enhanced the functions of the secretary-general. He was a bold man. The secretary-general must always fill a void when other organs of the U.N. cannot act."

In the current crisis, Perez de Cuellar felt it was only right to defer to a member country, the United States, conducting its own diplomatic initiative. The Security Council's resolution of April 3 asking Argentina to withdraw did not assign him a specific role.

Once the United States got out of the middle, however, Perez de Cuellar came forward to take up the slack, in the spirit of the Security Council resolution. His goal was not to be a mediator presenting formulas or an arbitrator imposing a settlement but to offer good offices, to be an honest broker, "to be ready with some alternative when they are in an impasse."

He had the advantage, he says, of having observed what worked and what didn't for Secretary of State Alexander Haig. With this in mind, he altered course, excluding from his discussions the issue of eventual sovereignty and concentrating on the "procedures" of ending hostilities and opening negotiation.

"Having dealt with many interlocutors, I find that human beings are all the same when they are defending their interests," he says. "A lot of emotion and a lot of distrust don't help, but both countries seem interested in a peaceful solution, and they are being as helpful as possible." Of the upbeat note he has regularly struck in this crisis, he observes: "I never lie. I have no right to raise expectations unreasonably."

Perez de Cuellar acknowledges that this crisis is not the ultimate test. The dispute is of a certain size, the violence has not become irreversible, the parties have reason to want a neutral to twist their arms, and the superpowers, not being directly involved, are letting him do his work.

A success would show the Reagan administration, which has found many occasions to denigrate multilateral diplomacy, that the U.N is not a "zoo," as one insider puts it. It would show what the same source terms the "jumpers up and down"--the Third World--and not only them, that the quiet ways of the secretariat can accomplish results that loud resolutions in the General Assembly cannot.

Perez de Cuellar and others at the big glass palace understand the importance of the event. It goes beyond his reputation and effectiveness. I would put it this way: if the U.N can't do this one, what can it do?