WHAT IS IT that keeps the Fourth of July flying so high while other national holidays droop, lose their color and fade into the bland category of "Hey, we get Monday off, don't we"? Why is it that more than 200 years after the occasion, huge numbers of Americans continue to commemorate the country's declaration of independence from the British crown by crowding together in mind-frying heat, exerting themselves to the point of sunstroke and risking life or limb igniting unreliable small explosives?

The answer, of course, is that the Fourth is the nation's renewable holiday, a celebration not just of its birth but of the thing that unites it more than ties of blood, culture or language: its democratic values. The exuberance with which Americans celebrate this day is evidence that these values continue strong, and that the people think it is important to reaffirm their faith in them.

They are old ideas that seem tame and overly simple in this era of scourging social revolutions: free elections, personal liberty. But as recent events in Korea, China, the Philippines and even the Soviet Union have shown, they can bring out a good crowd. They can also be profoundly unsettling to governments because they foster a cantankerous attitude in the populace, a reluctance to be pushed around by the state.

This week in our area, police have been watching for people who buy fireworks in Virginia, and arresting them when they cross into Maryland, where such things are illegal. Americans to the core, most of those arrested seem to be not ashamed but infuriated. "It's a rotten way to do business," said one man quoted in this newspaper. "It's a pretty sneaky thing to do," said another. "It's a shame that Maryland has taken it upon itself not to celebrate," said a fireworks merchant. We don't know what Jefferson would have said on this issue, but we suspect that the Fourth can do without the home fireworks; the important thing is that the citizens keep popping off.