Adhesion is the natural force of attraction between two materials that are to be glued together. The adhesive ensures that the microscopic roughness in the surfaces of the materials is evened out, thereby strengthening the bond. When this is combined with the internal hardening of the adhesive, i.e. the bonding together of the individual adhesive particles themselves, which is called cohesion, a strong connection is created by the gluing process.
Description of adhesives which harden in the absence of air and in contact with metal.