Newton used linseed oil as thermometric material and measured its change of volume against his reference points. He set as 0 on his scale "the heat of air in winter at which water begins to freeze" (Calor aeris hyberni ubi aqua incipit gelu rigescere), reminiscent of the standard of the modern Celsius scale (i.e. 0 °N = 0 °C), but he has no single second reference point; he does give the "heat at which water begins to boil" as 33, but this is not a defining reference; the values for body temperature and the freezing and boiling point of water suggest a conversion factor between the Newton and the Celsius scale of between about 3.08 (12 °N = 37 °C) and 3.03 (33 °N = 100 °C) but since the objectively verifiable reference points given result in irreconcilable data (especially for high temperatures), no unambiguous "conversion" between the scales is possible.[2]