ADHESION, in physics, the force which holds together two surfaces brought in con tact; distinguished from cohesion, the mutual attraction exerted by particles of the same body, and from affinity, since the particles ad hering remain unchanged. It is a force ex erted on each other by the molecules of the adhering bodies, and not to be confounded with mere mechanical contact due to pressure. The wetting of solid bodies is an instance.
It usually happens that when a solid and a liquid come in contact, a film of the liquid adheres to the solid too firmly to be detached, showing its adhesion to the solid to be stronger than the cohesion of its particles or the force of gravitation, as it can be removed only by forcible rubbing or evaporation. On the other hand, solutions are supposably cases where the adhesive force of solid and liquid overbalances the cohesive force of the solid, so that it loses its form and adheres particle by particle: but see SOLUTIONS, the true theory of which is keenly debated. The force of adhesion is measured by poising a metal plate on a bal ance, and then finding whet additional force is required to detach it from the surface of a liquid which does not wet it (otherwise it would be measuring the cohesive force of the liquid) nor act on it chemically. The phe nomena of capillary attraction depend on adhesion. Solid bodies also adhere to solids; most smooth surfaces will adhere; the smooth er the tighter; and two plates of polished glass laid together can hardly be parted without breaking them. If the solids are pressed to gether, it usually increases the adhesive force; but it depends but little on atmospheric pres sure. Friction is a looser kind of adhesion, which prevents surfaces moving freely on each other, and may result from gravitation or mechanical appliances. Plating, gilding, etc.,
also depend on adhesion. Soldering, the use of mortar, cementing, gluing, etc., are familiar applications of the principle, intermediary sub stances being employed, whose particles have at once great cohesion among themselves and great adhesion to each of the bodies to be joined. A familiar example is the splitting of a thin sheet of paper by pasting it between two sheets of cloth and pulling them apart after it has dried; the adhesion of paste to paper and cloth is so great that the paper fibres yield to it. Furthermore, air and other gases adhere to solids; a favorite children's experiment is to float a dry needle in a basin of water, it resting on a cushion of air; and when ther mometers are filled with mercury it has to be boiled in them to expel the air that adheres to the glass. Every material body, and every par ticle of such body in however fine division, is surrounded by its own atmosphere of con densed gases, which are an efficient factor in many physical and chemical phenomena; this property in comminuted bodies is called ad sorption, and in metallic substances is some times so avid that they grow red-hot.
In pathology, adhesion is sometimes applied to the closing of a wound. It occurs when two granulating surfaces are kept in contact that they may fuse and the wound unite by adhe sion. The pleura, pericardium and peritoneum often adhere when inflamed, and in inflamma tions of the vermiformis appendix, pelvic or gans, etc., more or less extensive adhesions sometimes occur, and special operations may be necessary to allow the free functioning of the organs.