Quincy ( ), known as Illinois's "Gem City," is a city on the Mississippi River and the county seat of Adams County, Illinois, United States. As of the 2010 census the city held a population of 40,633, up from 40,366 in 2000. During the 19th Century, Quincy was a thriving transportation center as riverboats and rail service linked the city to many destinations west and along the river. It was once Illinois' second-largest city, surpassing Peoria in 1870. The city holds several historic districts, including the Downtown Quincy Historic District and the South Side German Historic District showcasing the architecture of Quincy's many German immigrants from the late-19th century.