Aargau (ärˈgou) [key], Fr. Argovie, canton (1993 pop. 512,000), 542 sq mi (1,404 sq km), N Switzerland. Aarau is the capital. It is traversed by the Aare and Reuss rivers. Its fertile hills and valleys are used to raise fruit and cereal as well as for other agriculture and dairying. Electrical goods, textiles, cement, and precision instruments are principal manufactures. Baden and Rheinfelden are noted health resorts. Originally a Celtic settlement, the area was later occupied by the Romans and fell to the Franks in the 6th cent. The territory was taken (1415) by Bern from the house of Hapsburg and was governed by the Swiss cantons until 1798. In 1803, Aargau was admitted as a canton to the Swiss Confederation. Its population is mainly German-speaking and Protestant.