Why were the years between 1848 and 1850 a turning point?

write outlines to a few practice essay questions and there is one that is giving me trouble in particular. Could really use some help from somebody with some knowledge in Western Civ. This is the question: Between 1801 and 1911, the number of European cities with populations of more than 100,000 people increased from one to 36. What were the main causes and the primary effects, positive and negative, of this massive wave of urbanization? Who felt the negative effects most severely? Why were the years between 1848 and 1850 a turning point?