The cohesive force acts in different substances with different degrees of energy: in some its intensity is very great; but the sphere of its influence apparently very limited. This is the case with all bodies which are hard, strong, and brittle, which no force can extend or stretch in any perceptible degree, and which require a great force to break or tear them asunder. Such, for example, is cast iron, certain stones, and various other substances. In some bodies the cohesive force is weak, but the sphere of its action considerable. Bodies which are easily extended, without being broken or torn asunder, furnish examples of this. Such are Indian-rubber, or caoutchouc, several animal and vegetable products, and, in general, all solids of a soft and viscid kind. Between these extremes, the cohesive force may be observed in various degrees. In lead and other soft metals, its sphere of action is greater, and its energy less, than in the former examples; but its sphere less, and energy greater, than in the latter ones. It is from the influence of this force, and that of the repulsion, whose sphere of action is still closer to the component atoms, that all the varieties of texture which we denominate hard, soft, tough, brittle, ductile, pliant, &c. arise. After having been broken, or otherwise separated, the parts of a solid may be again united by their cohesion, provided any considerable number of points be brought into sufficiently close contact. When this is done by mechanical means, however, the cohesion is not so strong as before their separation, and a comparatively small force will be sufficient again to disunite them. Two pieces of lead freshly cut, with smooth surfaces, will adhere when pressed together, and will require a considerable force to separate them. In the same manner if a piece of Indian-rubber be torn, the parts separated will again cohere, by being brought together with a slight pressure. The union of the parts in such instances is easy, because the sphere through which the influence of cohesion extends is considerable; but even in bodies in which this influence extends through a more limited space, the cohesion of separate pieces will be manifested, provided their surfaces be highly polished, so as to insure the near approach of a great number of their particles. Thus, two polished surfaces of glass, metal, or stone, will adhere when brought into contact. In all these cases, if the bodies be disunited by mechanical force, they will separate at exactly the parts at which they had been united, so that after their separation no part of the one will adhere to the other; proving that the force of cohesion of the surfaces brought into contact is less than that which naturally held the particles of each together.