as the seat of secret Polish insurgent authorities. In January 1864, the Prussian police arrested a number of members of the Polish insurgent movement. The Unification of Germany in 1871 turned Breslau into the sixth-largest city in the German Empire. Its population more than tripled to over half a million between 1860 and 1910. The 1900 census listed 422,709 residents. In 1890, construction began of Breslau Fortress as the city's defenses. Important landmarks were inaugurated in 1910, the Kaiser bridge (today Grunwald Bridge) and the Technical University, which now houses the Wrocław University of Technology. The 1900 census listed 98% of the population as German-speakers, with 5,363 Polish-speakers (1.3%), and 3,103 (0.7%) as bilingual in German and Polish. The population was 58% Protestant, 37% Catholic (including at least 2% Polish) and 5% Jewish (totaling 20,536 in the 1905 census). The Jewish community of Breslau was among the most important in Germany, producing several distinguished artists and scientists. From 1912, the head of the university's Department of Psychiatry and director of the Clinic of Psychiatry (Königlich Psychiatrischen und Nervenklinik) was Alois Alzheimer and, that same year, professor William Stern introduced the concept of IQ. In 1913, the newly built Centennial Hall housed an exhibition commemorating the 100th anniversary of the historical German Wars of Liberation against Napoleon and the first award of the Iron Cross. The Centennial Hall was built by Max Berg (1870–1947), since 2006 it is part of the world heritage of UNESCO. The central station (by Wilhelm Grapow, 1857) was one of the biggest in Germany and one of the first stations with electrified railway services. Since 1900 modern department stores like Barasch (today "Feniks") or Petersdorff (built by architect Erich Mendelsohn) were erected. Following the First World War, Breslau became the capital of the newly created Prussian Province of Lower Silesia of the Weimar Republic in 1919. After the war the Polish community began holding masses in Polish at the Church of Saint Anne, and, as of 1921, at St. Martin's and a Polish School was founded by Helena Adamczewska. In 1920 a Polish consulate was opened on the Main Square. In August 1920, during the Polish Silesian Uprising in Upper Silesia, the Polish Consulate and School were destroyed, while the Polish Library was burned down by a mob. The number of Poles as a percentage of the total population fell to just 0.5% after the re-emergence