When director Brian De Palma adds the ingredient of violence to his movies, his unit of measurement is not the soupcon. He favors scenes like the one in his hugely successful ''The Untouchables,'' when Al Capone, played to reptilian perfection by Robert De Niro, hosts a black-tie dinner for his hoods. Peeved to the point of violence by an associate's behavior, Capone pulverizes the associate's skull with a baseball bat.

It is a stomach-turning scene, but before denouncing De Palma (as I have done for gratuitous carnage in movies like ''Scarface''), note that an episode very like the one depicted actually happened. De Palma's ''Untouchables'' is a correct and corrective depiction of squalid creatures who often have tapped a vein of unseemly American tolerance.

The tendency to ascribe virtues to gangsters resurfaced in the 1970s in Mario Puzo's ''The Godfather.'' Puzo's mobster was a sort of statesman, a law-giver outside the law. Played in the movie by Marlon Brando, there was something almost stately about the mafioso's physical movements, as there was about the rituals by which little people petitioned him for redress of their grievances.

Furthermore, a recurring theme of ''The Godfather'' is that there is a rough congruence between business values and gangster values. Gangsters are recognizable types -- profit-maximizers protecting markets and enforcing contracts.

It was in the 1920s that gangsters became some of America's first celebrities. Capone had a flair for quips (''I don't even know what street Canada is on''), political philosophy (''When I sell liquor, it's called bootlegging; when my patrons serve it on silver trays on Lake Shore Drive, it's called hospitality'') and even aphorisms (''You can get much further with a kind word and a gun than you can with a kind word alone''). But he was a grotesque brute and thus it is important to understand why, when he showed up at Wrigley Field, the crowd applauded.

In a brilliant stroke, De Palma opens his movie with a scene of Capone being barbered while toadying reporters eagerly gather morsels for their avid readers. Capone was a product of Prohibition, but the gangster as superstar was a product of, among other things, journalism and other publicity mechanisms that turned the 1920s into (the phrase is Frederick Lewis Allen's) ''the ballyhoo years.''

By now, Americans have, as it were, callouses on their minds. Americans are much more impervious than in the 1920s to manufactured hysteria. In the 1920s, the decade that produced the (to the undiscerning eye) dullest president (Coolidge) produced a steady stream of publicity extravaganzas. National magazines had mushroomed, advertising and publicity agents were honing their skills and, most important, radio had arrived. As Allen wrote, ''The national mind had become as never before an instrument upon which a few men could play.''

There was now mass production not only of material goods but of ideas, fads, publicity. There was an exponential growth in the power to make people famous. There was a new fabricated thing: the celebrity.

Celebrities came from the world of sports (there has never been a decade like it: Babe Ruth, Bobby Jones, Jack Dempsey, Gene Tunney, Red Grange, Knute Rockne, the Four Horsemen), daring (Lindbergh, Admiral Byrd), entertainment (Rudolph Valentino, Rudy Vallee) and crime (Capone, ''Legs'' Diamond and others).

Theories abound. Perhaps the nation's susceptibility to fads (Mah-Jongg, crossword puzzles) and fascination with celebrities had something to do with the dissatisfying aftermath of the war. Perhaps the peculiar attitudes toward gangsters had something to do with the enormous prestige and crude popularization of science, which seemed to teach that God is dead and man is degraded far below the angels.

Certainly the gangster was an urban cowboy, tapping the perennial American sympathy for the untamed outsider. His heroics (as they seemed to many people) were made possible by new technologies: the automobile that made escapes easy, and the availability of wartime weapons, such as submachine guns. And as Allen wrote part of the problem was ''the sheer size and unwieldiness of the modern metropolitan community, which prevented the focusing of public opinion upon any depredation which did not immediately concern the average individual citizen.''

Because that last point is still pertinent, it is notable that early in De Palma's movie a little girl is killed by a gangster's bomb, and late in the movie a baby in a carriage is caught in a horrifying cross fire. De Palma's demythologizing point is that real gangsters are not (as in ''The Godfather'') primarily a danger to other gangsters. They are enemies of the innocent, with no claim on any kind of admiration.