Cohesion and adhesion

When we consider the magnitude of the forces that must be applied to tear apart portions of solid bodies, it is evident that the particles composing the bodies must be mutually attracted, and by large forces. The particles are said to cohere, and the property is called cohesion. We know further that such forces must be effectively operative but over small distances, for a solid body, after being cut in two, will not re-cohere with cohesive forces that approach something like the original values even after the faces of the cut have been carefully polished.

By great refinement in polishing, and by pressing the surfaces together, still greater force can be developed, but the original value still will not be re-established.

Adhesion is a similar property, but involving unlike bodies in contact. Cohesions and adhesions are shown not only by liquids but also by solids.

An interesting illustration is found in the phenomena that occur in contacts between solids and liquids. Some liquids "wet" a given solid, but others do not, For example, an attempt to empty a clean glass vessel after tilling it with water leaves the entire surface of contact wet, since the water has adhered to the glass so that the forces of cohesion of its own particles were unable to tear it away.

This is not the only feasible example, as there are many other liquids with like adhesive properties.

Contrasting behaviour is shown by mercury, that does not "wet" clean glass. So, a beaker filled with mercury can be completely emptied, as the cohesion of the particles of mercury is greater than adhesion between mercury and glass.

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