COHESION, one of the species of at traction, denoting that force by which the parts of bodies stick together.
This power was first considered by Sir Isaac Newton as one of the properties essential to all matter, and the cause of all that variety observed in the texture of different terrestrial bodies. He did not, however, absolutely determine that the power of cohesion was an immaterial one, but that it might possibly arise, as well as that of gravitation, from the action of another. His doctrine of cohesion is thus expressed : "The particles of all hard homogeneous bodies, which touch one another, cohere with a great force ; to account for which, some philosophers have recourse to a kind of hooked atoms, which in effect is nothing else but to beg the thing in question. Others ima gine that the particles of bodies are con nected by rest, i. e. in effect, by nothing at all ; and others by conspirin motivms, s. c. by a relative rest among themselves. For myself, it rather appears to me that the particles of bodies cohere by an attrac tive force, whereby they tend mutually to. ward each other ; which force, in the very point of contact, is very great ; at little distances is less ; and at farther dis tances is quite insensible."
But, whatever the cause of cohesion may be, its effects are evident and cer tain. The different degrees of it consti tute bodies of different forms and proper ties. Thus, Newton observes, the parti cles of fluids which do not cohere too strongly, and are small enough to render them susceptible of those agitations which keep liquors in a fluid state, are most easily separated and rarefied into vapour, and make what the chemists call volatile bodies ; being rarefied with an easy heat, and again condensed with a moderate cold. Those that have grosser particles, and so are less susceptible of agitation, or cohere by a stronger attraction, are not separable without a greater degree of heat ; and some of them not without de composition.
Modern chemists have agreed to con sider the attraction of cohesion as the in strument of aggregation, or the union of similar compounds, and are careful not to confound it with the elective attrac tions, though there may, in strictness, be no difference between them. See CHE.