Congresses 2,690 History 1,185 Periodicals 641 Social conditions 432 Societies, etc 328 Politics and government 274 Data processing 237 Education 221 Social aspects 219 Heat 206 Economic conditions 205 Sources 194 Testing 192 Literature and society 187 Bibliography 186 Transmission 180 Materials 178 Science 178 Design and construction 173 History and criticism 168 Sociology 165 Study and teaching 161 Metals 150 Research 150 Geology 133 Mathematical models 131 Chemistry 126 Biography 123 Mathematics 121 Electronic data processing 116