Relates to liquids (excluding petroleum products) having a boiling point in the range 30 °C to 300 °C and are stable during distillation in atmospheric conditions. Specifies also the distillation apparatus. The principle consists in a) distillation of a test portion under carefully defined conditions in order to determine the temperature corresponding to the condensate volumes defined (plotting temperature as a function of volume) or b) under the same conditions to determine the volume of condensate when the thermometer indicates each of the distillation temperatures specified (plotting volume as a function of temperature).