periodicals 21 teaching aids 18 osobnosti 16 research 9 science 7 history 6 chemistry 4 more ... electrotechnics 4 environment protection 4 genetics 3 wastes 3 weeds 3 popularization 3 natural sciences 3 mechanical engineering 3 engineering 3 astronomy 2 biology 2 biotechnologies 2 chemical industry 2 ecosystems 2 power engineering 2 infection 2 lasers 2 aircrafts 2 medicine 2 fruit growing 2 brewing industry 2 food-processing industry 2 soil science 2 less ...