Cohesion depends upon the molecular forces which keep together the parts of bodies, and are insensible at sensible distances. In the case of a small body these forces are much more efficient in keeping the parts together than are the mutual gravitational attractions of the parts. On the other hand, in the case of a large body, such as the earth, gravitation is much more powerful in preventing rupture than cohesion. The term adhesion is generally, though quite unnecessarily, used when the cohering particles are dissimilar—as when water clings to glass.
After the parts of a body have been separated, so as to form an incoherent mass, the force of cohesion may be again brought into action by the application of pressure sufficient to force the particles close enough together. Thus two smooth, freshly cut pieces of lead can be made to adhere by slight pressure accompanied by a screwing motion. Also two very smooth slabs of marble adhere, if pressed together, so that considerable force must be exerted to separate them. Barton made a set of cubes with surfaces so true that, if twelve of them were piled one over the other, the whole series could be lifted by raising the upper one. Lead pencils are made by applying pressure to powdered graphite, so as to make it cohere.