COHESION, the force by which the vari ous particles of the same material are kept in contact, forming one continuous mass. Its action is secn in a solid mass of matter, the parts of which cohere with a certain force which resists any mechanical action that would tend to separate them. In different bodies it is exerted with different degrees of strength, and it is measured by the force necessary to pull them asunder. Cohesion in liquids is very much weaker, the parts being disjoined with much more facility; and in substances existing in the gaseous form it is entirely overcome, the particles repelling instead of attracting each other.
Cohesion in bodies is weakened or over come by two general causes— by the repulsion communicated by heat, or by the attraction which may he exerted by the particles of one body on those of another.
Heat communicated to a solid body always diminishes the force with which the attraction of aggregation or cohesion is exerted; if the heat be increased to a sufficient extent the co hesion is so far weakened that the body passes into the liquid fortn; and if carried still ther, the attractive force is still more completely overcome and the body passes into the gaseous state. See GASES, GENESAL PROPERTIES OF.
The same effects are produced by the exer tion of that attraction which unites the particles of one body with those of another. If a liquid' be poured on a solid, it often happens that their mutual attraction is sufficiently powerful to overcome the cohesion of the solid; its par ticles are consequently disunited, to combine with those of the liquid, and it entirely dis appears. This forms the process of solution
(q.v.).
When these powers are withdrawn, cohesion resumes its force, but with results which are different, according to the circumstances under which this happens. When the attraction of aggregation is suddenly and forcibly exerted the particles are united, in general, indiscrimi nately, and according to no regular law. If a body which has been melted is suddenly cooled to a sufficient extent, it becomes solid and forms a mass of no regular structure or figure; or if its cohesion has been suspended by the chemical attraction exerted by another body toward it, and if this attraction suddenly cease to operate, tir force of cohesion is re sumed, and the solid substance appears in the form of a powder. This latter case forms the chemical operation denominated precipitation (q.v.). But if the force of cohesion is exerted more slowly the particles are united, not in discriminately, but usually with regularity, so as to form masses of regular structure and figure, bounded by plane surfaces and determi nate angles. This forms the operation of crys tallization; and such masses arc denominated crystals (q.v.). See STRENQT11 OF MATERIALS.