The atmosphere in Berlin changed dramatically toward the end of Riemann's second semester there. After Louis Philippe fell from power in France in February 1848, a great wave of demands for political reform swept through Europe, placing renewed pressure on Prussia's King Friedrich Wilhelm IV. Fearing he might be the next to fall, he called his soldiers to Berlin to maintain order. This soon led to heightened tensions and eventually a bloody open battle that cost some 200 Berliners their lives. Riemann was an eye witness to much of this and reported to his family on what he saw or heard. When he returned for his second semester, Berlin was still shaking from these events, which eventually ended in defeat for the reformers. Starting in the summer of 1849, Riemann took up studies again in Göttingen, where working under Wilhelm Weber, he became increasingly interested in geomagnetism and electrical phenomena. In 1851, he submitted his dissertation, in which he described a new foundational approach to the theory of functions of a complex variable. His promotion to Dr. Phil. was complicated by the fact that he was required to defend theses in a formal disputation conducted in Latin. Afterward, he prepared his postdoctoral thesis on a topic related to Dirichlet's earlier investigations on Fourier series representations. During this entire period, Riemann steeped himself in experimental and theoretical physics, while pursuing philosophical ideas related to the works of Herbart.