What made colonialism popular in Europe in the mid-19th century? How did the colonial powers legitimize colonialism? Elaborate the policies put in place by the colonial state that transformed the economic life of the subjects?

Colonialism and imperialism appealed to many European leaders in the mid to late nineteenth century for a number of reasons. Let us look at three important motives for imperialism.
First, Europe was in the midst of the Industrial Revolution during this period, and there was a demand for new markets to purchase European manufactured goods. Many people actually argued that if no such captive markets could be found, then the pace of industrialization would lead to overproduction. This, in turn, would cause repeated and severe economic recession. So markets, especially in places like China, were a major incentive for imperialism. 
Second, many Europeans argued that imperialism was actually an obligation. They believed it was incumbent upon Christians to spread their religion, their culture, and technology to people who were deemed less advanced around the world. This ideology, famously described as the "white man's burden" by British writer Rudyard Kipling, was a powerful one in nineteenth century Europe.
Third, many Europeans also believed that they were culturally, genetically, and racially superior to other peoples. Thus it was seen as only natural that they would conquer other, presumably "inferior" peoples. This idea is called Social Darwinism, and it also argued that international relations (i.e., between European nations) were a sort of death struggle in which only the most powerful nations would survive and flourish. So a competition for colonies was held to be essential to the well-being of European powers.
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