Liquid Penetrant is a nondestructive means of locating surface discontinuities based on capillarity or capillary action.  A penetrating liquid is applied to the surface of the test specimen, and is allowed to seep into any surface discontinuities.  If the discontinuity is small or narrow, such as cracks or pinholes, capillarity assists the penetration.  After sufficient time has passed for the penetrant to enter the discontinuity, the penetrant on the surface is cleaned off of the part.  Capillary action is again employed to act as a blotter to draw penetrant from the discontinuity.  To ensure visibility, the liquid penetrant contains either a colored dye easily seen in white light or a fluorescent dye visible under black ultraviolet light.