COHESION is the name to given that species of attraction (q.v.) by which the particles of matter are held together so as to form bodies (see ADHESION), and its measure is the resistance which bodies offer to any mechanical force tending to separate their parts. In gaseous bodies, C. is altogether wanting; their atoms even repel one another. In liquids, notwithstanding the ease with which the particles slide on one another, the opera tion of C. is distinctly seen in the formation of drops. C. is strongest in solids; and degrees of C., in this case, are much the same thing as degrees, of solidity. It is the force of C. that constitutes the strength of materials (q.v.). After the particles of a body have been completely separated, it is found that through C. they will reunite, lf pressed sufficiently close together. Two clean, smooth, freshly cut pieces. of lead placed together, will cohere so as to require a very considerable force to Separate them ; and it has not unfrequently happened in plate-glass manufactories, that polished plates of glass have cohered so completely that they have been cut and worked as a single piece.
If the particles of matter had no property in relation to one another, except their mutual impenetrability, the universe, it has been said, would be like a mass of sand, without variety of state or form. As it exists, however, it demonstrates the cross-action of several universal properties of matter. Among those which most affect its state and form, are heat and cohesion. It may be said that bodies assume the solid, liquid, or aeriform states, just according to the proportion that the C. of their particles bears to those forces which, like heat, tend to -separate them. Sec HEAT. Upon modifications of the cohesive force, and its relations to other molecular forces, would seem to depend such properties as elasticity, brittleness, ductility, etc.