These adhesion plates at Hampden-Sydney College in Virginia are unmarked, but similar ones are described in the 1856 Pike catalogue as: "Adhesion Plates: These consist of two plates of glass [brass in this case]... the flat surfaces ground perfectly true. On pressing these together, they can be separated only by the exertion of considerable force, the power required being in proportion to the area of the plates. About 2 1/2 to 3 inches in diameter is the usual size. Price $1.50." Left unsaid is the fact that atmospheric pressure on the outside of the plates supplies the force to keep the plates together.