History 356 Slave trade 269 Iron industry and trade 163 Slavery 159 Tariff 89 Steel industry and trade 80 Commerce 73 more ... Coal trade 67 Free trade 66 Protection 63 Free trade and protection 60 Book industries and trade 51 Fur trade 51 Clothing trade 49 Petroleum industry and trade 46 International trade--18th century 42 Labor unions 42 Protectionism 41 Enslaved persons 38 African Americans 34 Sugar trade 33 International trade--19th century 30 Economic conditions 29 Esclaves 29 Social conditions 27 Merchants--Pennsylvania--Philadelphia--18th century 26 Grocery trade 25 Colonies 24 Histoire 24 Antislavery movements 23 Food industry and trade 23 Streets 22 Lumber trade 21 Coal mines and mining 20 Emancipation 20 Railroads 20 Slave-trade 20 Description and travel 19 Indians of North America 19 Politics and government 19 less ...