Arriving at their favorite sleeping spot in Paris one chilly night last month, a group of derelicts began settling down on a sidewalk grid that spewed hot air from the Metro underground. Suddenly a figure dressed in black appeared out of the darkness and handed each of them what looked like a large plastic bag. As the bewildered bums looked on, he attached the open end of one bag to the sidewalk air vent with small metal hooks. Voil`a! It ballooned into a small, conical, one-man tent. Catching on quickly, the grateful men set up their individual shelters and settled down for a comfortable sleep, while Hans Walter Mueller, 39, the world's leading promoter of inflatable structures, padded off into the night.

Something Magic. Mueller, a slight man who habitually wears black trousers, a black sweater, a black velvet jacket and a picador's black hat, does not stop at enclosing Parisian derelicts in plastic bubbles. Among the larger bubbles he has designed and built are an inflatable theater that seats 800 people and an inflatable church that conveniently folds down to a 2-ft. by 4-ft. package after services. His passion for bubbles has also hit him where he lives: a shimmering, red-and-white candy-striped vinyl bubble house at the edge of a forest in La Ferte-Alais, 28 miles south of Paris.

German-born and a trained architect, Mueller got into the bubble business almost by accident. He came to Paris in 1961 on a French government scholarship, worked for traditional architectural firms and began to experiment with light as a means of changing an environment. He soon had his own room in a light show in the Paris Museum of Modern Art, where he could be found drinking wine and talking with visitors on nights when the museum was open late. By then Muller's ideas had begun taking on a new shape; he wanted different materials on which to project his light designs. So he began concentrating on inflatables, which are light, easy to work with, movable and cheap. Now they have taken over his life. Says Mueller: "The inflatable for me is something magic--like the light, the sound, the sun, the sea."

Mueller's inflatable house is indeed unique; it has 210 square meters of floor space, the ceiling rises nearly 16 ft. at its apex, and the entire structure is held up by a constant flow of air pumped through a plastic umbilical cord by a small electric motor outside. Shadows from trees and clouds dance across the walls and roof, changing shape as the afternoon sun dips toward the horizon. When Mueller tires of the shifting shadows, he projects pictures of mountains, oceans and forests on the walls. In warm weather he pipes water to the roof, where it forms an enlarging puddle that depresses the vinyl and creates a natural swimming pool. (To get rid of the water, Mueller simply turns up the pressure inside the bubble; the roof rises and the pool empties down the side of the house.)

Mueller's inflatable also is his workshop, where he is finishing plans for a bubble aviary for a zoo, a bubble house for a neighbor (cost: about $6,000), and was working on a bubble to fit over the helicopter on the deck of the late Aristotle Onassis' yacht. He is also negotiating with Algeria about building an entire inflatable resort town. In fact, there is nothing that Mueller would not consider enclosing in a bubble to improve the human condition. "Inflatables give you a sense of self-reliance," he says. "There are no walls to hide behind."

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