Cohesion, that kind of attraction by which the particles of bodies are held together, as the molecules of water, of iron, or of stone. It is manifested in a high degree in solids, less in liquids, and but very little or not at all in gases. The molecules of which bodies are composed would seem, from the various phenomena observed in nature and in experiments, to be under the influence of two opposing forces, which under varying conditions alternate with each other in preponderance. One of these forces is molecular attraction, and the other is molecular repulsion. The first is exerted only at inappreciably small distances, but the laws by which it is governed are not known. Repulsion is exerted at greater distances, and is principally controlled by the action of heat, increasing as the heat increases, the abstraction or addition of this agent being usually sufficient to cause matter to assume either the solid, liquid, or gaseous form. Molecular attraction manifests itself in three ways, by cohesion, by adhesion, and by chemical affinity.

The degree of cohesion in the same solid depends much upon the arrangement of its particles, as may be observed in the different degrees of hardness between tempered and untempered steel, and the difference in tenacity of metals in ingots or in hammered or rolled plates. The distinction between cohesion and adhesion is not always easily to be established. Adhesion is commonly spoken of as existing between the particles of dissimilar bodies, but it is more accurately defined by calling it that molecular force which holds two distinct bodies together, whether they are of the same substance or not. If two pieces of lead have plane surfaces cut upon them, and these are firmly pressed together, they will continue to be so held by the force of adhesion (independently of atmospheric pressure), and not of cohesion, because that term cannot strictly be applied except when such a union has taken place as to render the structure continuous between the original bodies. To consider, also, cohesion as only capable of existing between particles of the same kind, would lead to conclusions that cannot well be maintained, because in an alloy of two or more metals, even when they are not mingled in the proportions of their atomic weights, it seems proper to consider the particles as being held together by the force of cohesion.

As has been intimated, the laws which govern molecular attraction are so imperfectly understood that the relations between cohesion, adhesion, and chemical affinity cannot be stated. The last named force is exerted between atoms or molecules of matter which are not of the same kind; it is manifested with more intensity than is observed in the phenomena of cohesion and adhesion, and is also followed by a more intimate union of the particles, by which the formation of a new body, unlike in its physical properties either of those of which it is composed, is accomplished. Heat exerts a remarkable influence upon these forces, and indicates the differences which exist in their nature. Adhesion does not seem to be so uniformly diminished by its action as cohesion, for the adhesion between substances is often increased thereby, while cohesion is constantly diminished. Chemical affinity is also generally increased with the increase of heat, certainly while union is taking place, whatever may be its effect upon the compound after union; but that heat has the power of diminishing the affinity with which the atoms of a compound are held together is shown by its decomposing action on some of the metallic oxides, as those of silver and mercury.

The manifestations of chemical affinity between two uniting bodies are no doubt often heightened by the application of heat, in consequence of its diminishing the cohesion or increasing the repulsion between the molecules of each of such uniting bodies. The conversion of water into steam is an example of the neutralization of the force of cohesion by the application of heat, without any appreciable diminution of the force of chemical affinity. The cohesive force of the atoms or the molecules of bodies depends on their distance from one another, decreasing as the distance increases, and disappearing entirely when that becomes sensible; but the law by which the decrease takes place is not known. It has been questioned, however, whether molecular attraction may not follow the law of attraction of gravitation, varying inversely as the squares of the distance; or, in other words, whether the two are not modifications of the same force. Under this assumption the explanation of the cessation of cohesive attraction at sensible distances is furnished by the obvious fact that any sensible distance is almost infinitely so much greater than the distance between the centres of the adjacent atoms or molecules of a solid or liquid, that the difference in the attractive forces becomes practically infinite.

The question as to the amount of cohesive force which exists between the molecules of liquids is attended with difficulty. The fact that water will boil in a vacuum at a low temperature might suggest the inference that there exists no positive cohesive force between its particles; but the formation of spherical drops of water is opposed to such a conclusion. Again, when a liquid boils in the open air it is usually considered that the repulsion between the molecules is sufficient to overcome the pressure of the atmosphere, and therefore it would appear as if there were no cohesive attraction between them; but the irregularity with which ebullition takes place under certain circumstances, as when the air which is usually contained in water has been expelled, would seem to confirm the opinion that, in reality there are present both attractive and repulsive forces, which alternate in predominance; the attractive force predominating when the molecules are within certain distances, especially when they are comparatively at rest, and the repulsive force predominating when the distances are deranged by motion, or increased by the intrusion of particles of vapor or air. (See Boiling Point.) Moreover, water may be considerably reduced in temperature below its freezing point without congelation taking place, when a jar given to the vessel, or a pebble or crystal dropped into it, will cause the immediate manifestation of intense cohesive force and the formation of ice.

It would therefore appear that the molecules of a liquid may have slight cohesive attraction for each other, even when the temperature is sufficient to cause repulsion between the particles of its vapor; and also that when the freezing point is reached, the cohesion may not be sensibly increased until some disturbing cause operates. The modifications of cohesive attraction are the cause of those different properties of bodies which are called tenacity, hardness, ductility, and elasticity, and will be treated under their appropriate heads, and in the article Strength of Materials.