History 388 Trade-unions 292 Slave trade 229 Iron industry and trade 156 Slavery 153 Tariff 92 Labor and laboring classes 79 more ... Steel industry and trade 77 Coal trade 67 Free trade 65 Protection 64 Free trade and protection 63 Book industries and trade 51 Slave-trade 50 African Americans 48 Fur trade 48 Clothing trade 45 Petroleum industry and trade 45 International trade--18th century 42 Protectionism 42 Biography 38 Commerce 38 Slaves 37 Periodicals 34 Employment 33 Social conditions 32 Sugar trade 32 Economic conditions 31 Railroads 31 International trade--19th century 30 Merchants--Pennsylvania--Philadelphia--18th century 26 Antislavery movements 25 Clothing workers 25 Colonies 24 Congresses 23 Grocery trade 23 Societies, etc 23 Streets 22 Coal mines and mining 21 Lumber trade 21 less ...