The capillary rise of oil in vertical strips of impregnated paper obeys the same law as the rise of liquids in capillary tubes of circular cross section. From the viscosity and surface tension of an oil and its rate of rise in a given paper, it is possible to compute the ``effective capillary radius'' of the capillary pores of the paper. The rate of rise in a calibrated paper affords a convenient means of measuring the surface tension when the viscosity of the liquid is known. The rate of penetration of an oil into a paper is directly proportional to the ``penetrativity'' of the oil as defined in the theory of capillary action. The penetrativities of nine insulating oils and the effective capillary radii of seven different papers have been determined. A uniform relationship between the effective capillary radius and the Gurley air resistance of the paper is indicated.