When a thin layer of a substance adheres strongly to another, holding to it on a molecular level as if it were glued to it. This situation is more likely to occur with substances in a gelatinous state and with the receiving surface absolutely clean. Often a truly adsorbed layer becomes so firmly attached to the receiving surface that it can be removed only by vigorous abrasion. One example of adsorption occurs in lithography, with the crayon or tusche on the stone or plate so that an adsorbed layer of fatty acid is in position to receive the printing ink. Also see occlude.