A Look Back at the Ehrenfest Classification. Translation and Commentary   of Ehrenfest's 1933 paper introducing the notion of phase transitions of   different order
  A translation of Paul Ehrenfest's 1933 paper, entitled "Phase transitions in the usual and generalized sense, classified according to the singularities of the thermodynamic potential" is presented. Some historical commentary about the paper's context is also given. 
Tilman Sauer
physics.hist-ph