History 348 Slave trade 263 Iron industry and trade 159 Slavery 155 Tariff 89 Steel industry and trade 76 Commerce 71 more ... Coal trade 67 Free trade 66 Protection 63 Free trade and protection 60 Fur trade 51 Clothing trade 46 Petroleum industry and trade 46 Book industries and trade 45 Labor unions 42 Protectionism 41 International trade--18th century 40 Enslaved persons 38 Sugar trade 33 African Americans 32 International trade--19th century 30 Economic conditions 29 Esclaves 27 Social conditions 27 Grocery trade 25 Merchants--Pennsylvania--Philadelphia--18th century 25 Colonies 24 Antislavery movements 23 Food industry and trade 23 Streets 22 Lumber trade 21 Coal mines and mining 20 Emancipation 20 Histoire 20 Railroads 20 Slave-trade 20 Indians of North America 19 Biography 17 Description and travel 17 less ...