'I'M Forever Blowing Bubbles" could have been the theme song of French painter Jean-Simeon Chardin."They all look alike," said the puzzled woman in pink as she stared at three paintings on the walls of this summer's Chardin show at the National Gallery of Art. Indeed, these three versions of Chardin's celebrated "Soap Bubbles" did resemble each other almost as closely as triplets. But each had come from different museums: one from the National Gallery's permanent collection, another from the Los Angeles County Museum of Art, and the last from New York's Metropolitan Museum of Art. Behind the bubbles is a bit of sleuthing. Art experts have used radiographs (X-rays of the works) to examine the process of creating these triplicate paintings. The 18th-century French artist, famous for his scenes of everyday life, chose the "Soap Bubbles" theme to symbolize the fleeting quality of human occupations and love. Prints of the radiographs of the paintings are hung in the entry room of "Soap Bubbles of Jean-Simeon Chardin." The black and white anatomies of the paintings reveal that there was apparently a fourth Chardin "Soap Bubbles," the prototype on which the other three are based. The radiographs that cover the paintings like a screen door indicate the process of what happened. The two major figures in all three paintings - a young man blowing a translucent, huge bubble out of the end of a straw while a little boy watches in fascination - are consistent in size and style. The radiographs also reveal that there are not pentimenti or revisions in composition in any of the three. Gumshoeing along, museum experts have also found that another of Chardin's works, "The Game of Knucklebones," (so similar that he often paired them in shows), does indicate considerable pentimenti added to the painting that are highlighted by the radiograph. This evidence reinforces what the "Soap Bubble" radiographs suggest: that there was a prototype painting, now lost (perhaps Chardin kept it for himself), which was a model for the three paintings in the show. It is also an example of the self-copying practiced by the French artist. In the painting, a young woman plays the game known as "Knucklebones," tossing a small ball in the air. If, as Charles Caleb Colton once wrote, "Imitation is the sincerest of flattery," Chardin must have had reasons other than ego for imitating his own works. His decision to recycle his art may have come from the fact that he labored long and hard over his paintings, taking time to brush on layer after layer as foundation. He may also have been pressured by those who commissioned his paintings. But it was a technique that many artists of the time used, finding an important image and then sacrificing originality to provide variations on that theme. The Washington "Bubbles," a large vertical painting, apparently resembled the lost original except for a honeysuckle vine that was painted in at the left at a later time. The Los Angeles "Bubbles" is smaller, vineless, and more horizontal, and the Met's "Bubbles," also smaller, has most of the vine cropped out. According to the show's brochure, the Washington painting "was enlarged by stretching strips of canvas on all sides." It also suggests that "the Los Angeles picture was made smaller by folding ove r the edges of the canvas," creating surface abrasions visible by radiograph. In addition, the radiograph of the New York painting indicates the picture "was somewhat larger and had been cut at the edge." The triple-play "Soap Bubbles" pictures have companion portraits by Chardin, including "The House of Cards,The Young Governess," and other works. At a press briefing J. Carter Brown, director of the National Gallery, noted that "There's always a lot of chat about museums being wrapped up in doing blockbuster shows...." But here "we've got a fabulous collection of Chardin," he says, and we thought that "starting to breathe that heady French 18th-century air would be very special." The show has already appeared at the Los Angeles County Museum of Art, which organized it, and at the Met. It will run through Sept. 2.