The most characteristic properties of solid bodies are the following: Hardness, tenacity, malleability, ductility, and crystalline form. Hardness is the resistance which a body offers to being scratched. Tenacity is the resistance offered by a body to a separation of its parts. Malleability is that property of a body which makes it capable of being rolled into sheets. Ductility is capacity for being drawn into wire, and crystalline form is the property which causes it to assume regular shapes.
As will be observed, these peculiarities are closely dependent upon cohesion and adhesion. By the former we understand the force which holds together the similar molecules of a substance; and by the latter, the force which unites the surfaces of different materials. Familiar as we are with these two agencies, their nature is not yet understood. We can easily discover that they are very dependent upon heat, by the application of which most solids pass from the stable form, to one in which, instead of cohesive force between the molecules, there is repulsion; as in the conversion of ice into water, and then into steam.
This movement of molecules is also dependent upon pressure. The most interesting illustration of this is seen in the action of glaciers. It has been ascertained that the melting temperature of ice lowers one two hundred and fiftieth of a degree for every fifteen pounds of pressure to the square inch.
The immense superincumbent mass of ice must, in many places, set free so much latent heat that a portion of the ice melts, so that here and there cells and liquid veins would be opened in the interior of the glacier. But the particles which separate these thin layers of water would almost immediately close up. This is the brilliant demonstration of Prof. Tyndall, who has given the operation the name of “regelation.” It has been thus described: “This phenomenon takes place at every point in the thickness of the glacier. Particles of ice approach one another, and unite across little veins of water, which permeate it in every direction; fresh liquid films are formed under the pressure from above; fresh unions take place between the divided morsels of ice; and, by this continual process of change, the air contained in the mass of that which once was snow, is gradually expelled. Thus it happens that the whole mass ultimately assumes an almost perfect transparency and a beautiful azure color.”