In the half century after the Civil War, the population of the United States quadrupled, but the urban population soared sevenfold. In 1870 one in five Americans lived in cities with a population of 8,000 or more. By 1900 one in three resided in cities of this size. In 1870 only Philadelphia and New York had populations over half a million. Twenty years later, in addition to these two cities, Chicago’s population exceeded 1 million; St. Louis, Boston, and Baltimore had more than 500,000 residents; and Cleveland, Buffalo, San Francisco, and Cincinnati boasted populations over 250,000. Urbanization was not confined to the Northeast and Midwest. Denver’s population jumped from 4,700 in 1870 to more than 107,000 in 1890. During that same period, Los Angeles grew nearly fivefold, from 11,000 to 50,000, and Birmingham leaped from 3,000 to 26,000. This phenomenal urban growth also brought remarkable physical changes to the cities, as tall buildings reached toward the skies, electric lights brightened the nighttime hours, and water and gas pipes, sewers, and subways snaked below the ground.