Parameters influencing the penetration coefficient λ were experimentally investigated by causing various liquids to flow in horizontal, straight glass capillaries of different radii. The conductivity of the capillary tube remains constant during the flow and is defined in terms of penetration coefficient, density, gravity, capillary radius, surface tension, and contact angle. The relationship between the square root of time and the penetration distance was found to hold true for the whole period studied. The experimental and theoretical values of both penetration coefficient and conductivity compared favorably and verified the validity of Poiseuille's law in unfilled capillaries. Theoretical proportionality between the square of the penetration coefficient and the capillary radius, on the one hand, and liquid properties in terms of α cos αα/μ, on the other, was found to be experimentally true. Experimental values of the penetration coefficient matched those calculated theoretically from handbook data when using the contact angle calculated from the capillary height reached by the advancing meniscus.