Stability in structures is usually secured by lowering the center of gravity in one of two ways: either by broadening the base or by making it of heavy materials.
Specific gravity is the weight of a substance as compared with an equal bulk of something taken as a standard; water having been selected as the standard for solids and liquids, and air for gases.
Gravity furnishes more units of measure of various kinds—weight, work, heat, tenacity—than any other force of nature.
It will be remembered that Physics is that branch of science that considers the general properties of matter, and the character of those forces which affect matter without destroying its molecule. It includes many subdivisions. In addition to those already mentioned, we find Molecular Attraction, or the operation of forces that act at insensible distances; Hydrostatics, which treats of liquids at rest; Hydraulics, of liquids in motion; Pneumatics, of gases; Machines, of means for applying force; Acoustics, of the laws of sound; Heat; Light; and Electricity.
As many physical properties have been mentioned in the articles on Air, Water, and Fire, they will not now be considered. Our discussion here applies more especially to those substances which, at ordinary temperatures, are solid.
Ex.—A body buoyed up in water displaces its own weight of the liquid. The glass is nicely graded, and as the water rises in the vessel, the registration at once indicates the amount of water displaced. This proves the truth of the “Law of Archimedes”[9], ascertained while he was investigating the problem of the golden crown.
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.