@book{BarthelDuvinage1999,
  author    = {Barthel, Helmut and Duvinage, Brigitte},
  title     = {R{\"u}benzuckergewinnung experimentell behandelt},
  year      = {1999},
  language  = {de}
}
@book{Barthel1999,
  author    = {Barthel, Helmut},
  title     = {Auch Clor entz{\"u}ndet spontan brennbare leichtfl{\"u}chtige Fl{\"u}ssigkeiten},
  year      = {1999},
  language  = {de}
}
@book{BarthelDuvinage1999,
  author    = {Barthel, Helmut and Duvinage, Brigitte},
  title     = {Ozon : Informationsmaterial zur Fortbildung von Chemielehrern},
  publisher = {Gesellschaft Deutscher Chemiker},
  address   = {Frankfurt am Main},
  year      = {1999},
  language  = {de}
}