A classification of phase transitions in terms of their thermodynamic properties put forward by the Dutch physicist Paul Ehrenfest (1880–1933) in 1933. A first-order phase transition is a phase transition in which the first derivative of the free energy is discontinuous. In a first-order phase transition there is a nonzero change in the value of the enthalpy, entropy, and volume at the transition temperature. Melting and boiling are examples of first-order phase transitions. In a second-order phase transition, the first derivative of the free energy is continuous but its second derivative is not continuous. In a second-order phase transition there is no jump in the value of the enthalpy, entropy, and volume at the transition temperature. Examples of second-order phase transitions include the transition to ferromagnetism and order–disorder transitions in alloys. Research on phase transitions since 1933 has modified the Ehrenfest classification into first-order phase transitions and continuous phase transitions.