Aix-la-Chapelle` (103), in Rhenish Prussia, one of the oldest cities in Germany, made capital of the German empire by Charlemagne; derives its name from its mineral springs; is a centre of manufacturing industries and an important trade; is celebrated for its octagonal cathedral (in the middle of which is a stone marking the burial-place of Charlemagne), for treaties of peace in 1668 and 1748, and for a European congress in 1818.