docs: def vapour_pressure(Temperature, element):
      r"""Return the vapour pressure of rubidium or cesium in Pascals.
  
      This function receives as input the temperature in Kelvins and the
      name of the element.
  
      >>> print vapour_pressure(25.0 + 273.15,"Rb")
      5.31769896107e-05
      >>> print vapour_pressure(39.3 + 273.15,"Rb")
      0.000244249795696
      >>> print vapour_pressure(90.0 + 273.15,"Rb")
      0.0155963687128
      >>> print vapour_pressure(25.0 + 273.15,"Cs")
      0.000201461144963
      >>> print vapour_pressure(28.5 + 273.15,"Cs")
      0.000297898928349
      >>> print vapour_pressure(90.0 + 273.15,"Cs")
      0.0421014384667
  
      The element must be in the database.
  
      >>> print vapour_pressure(90.0 + 273.15,"Ca")
      Traceback (most recent call last):
      ...
      ValueError: Ca is not an element in the database for this function.
  
      References:
      [1] Daniel A. Steck, "Cesium D Line Data," available online at
          http://steck.us/alkalidata (revision 2.1.4, 23 December 2010).
      [2] Daniel A. Steck, "Rubidium 85 D Line Data," available online at
          http://steck.us/alkalidata (revision 2.1.5, 19 September 2012).
      [3] Daniel A. Steck, "Rubidium 87 D Line Data," available online at
          http://steck.us/alkalidata (revision 2.1.5, 19 September 2012).
      """
      if element == "Rb":
          Tmelt = 39.30+273.15  # K.
          if Temperature < Tmelt:
              P = 10**(2.881+4.857-4215.0/Temperature)  # Torr.
          else:
              P = 10**(2.881+4.312-4040.0/Temperature)  # Torr.
      elif element == "Cs":
          Tmelt = 28.5 + 273.15  # K.
          if Temperature < Tmelt:
              P = 10**(2.881+4.711-3999.0/Temperature)  # Torr.
          else:
              P = 10**(2.881+4.165-3830.0/Temperature)  # Torr.
      else:
          s = str(element)
          s += " is not an element in the database for this function."
          raise ValueError(s)
  
      P = P * 101325.0/760.0  # Pascals.
      return P