The cooling curves of silver cylinder quenched in several samples of pure hydrocarbon prepared by the fractional distillation and other chemical operations from mineral oils were measured. The results are as follows: Among the oils of the same series, the larger the molecular weight is, i.e. the more the flushing point and the viscosity rise, the higher are the characteristic temperature and the beginning temperature of the convection stage. Among oils of nearly the same molecular weight, the oils consisting of hydrocarbons not of paraffin series have higher characteristic temperatures, higher begining temperatures of the convection stage and higher cooling velocity in the convection stage compared to the oils consisting of hydrocarbons of pure paraffin series. It seems that the cause of these facts is attributed to the vapor pressure, the boiling point, the ability of adsorption of constituent molecules and the temperature coefficient of viscosity of oils.