One hundred and seventy-five years ago the Erie Canal opened and New York—not Boston, not Philadelphia—became America’s gateway to the burgeoning West. By the Civil War it was the largest city in America, exercising a cultural hegemony it enjoys to this day. “Art and the Empire City: New York, 1825–1861,” the big, handsome new show running at the Metropolitan Museum of Art until January 7, 2001, charts that rise in more than 300 works gathered from 84 lenders. Among them: a maple and rosewood cabinet by Gustave Herter; a compote from the immigrant glassmaker Christian Dorflinger; a daguerreotype of the loyal city-singer Walt Whitman; and a wonderful 1855 view looking south across Forty-second Street and the reservoir that once stood on the present-day site of the New York Public Library.