There's preservation and there is pickling. Some things show up better under glass, and others begin to turn.

The intention of the American Dance Machine is to keep a living archive of American popular dance, and its 1978 recreation of several Broadway production numbers showed a new audience something of the delight of those achievements. (That the next year brought so many full-scale, all-out-lavish productions of old shows does not take away from the contribution.) But this year's American Dance Machine show, "Steps in Time," now at Ford's Theater, is not quite so successful.

The idea here is to preserve successful dance duos, from Broadway shows and from nightclub acts. But however sensational they were originally, bursting out in mid-musical or under dazzling spotlight, they look a little pathetic when placed back to back, as it were. "Dance can be anything," or "so many things," the dancers report as they introduce one another in voices breathless from their own efforts. True enough. But the typical light dance duo seems to have a very limited emotional range.

There is cute, and there is passionate. Cute can be coy, as in the Charleston number from "The Boy Friend," or it can be overly direct, as in "Moon Faced, Starry Eyed!" from "Street Scene." Passionate, whether it's supposed to be latiny or balletic, is always the same, with the pained look on the face and the sudden clutches and de-clutches.

This is not to say that these aren't good dances in their places. But taken out of place, and put into a collection, they lose the attractiveness of their natural settings.