There is one peculiar thing, that tenacity varies with the form of the body. A solid cylindrical body has a greater strength than a square one of the same size; and a hollow cylinder more tenacity than a solid one. This principle is well known in the bones of animals, in the feathers of birds, and in the stems of many plants.
In almost every metal tenacity diminishes as the temperature increases.
Ductility.—This is a property whereby a metal may be drawn out to form a wire. Some metals,[p. 81] like cast iron, have absolutely no ductility. The metal which possesses this property to the highest degree, is platinum. Wires of this metal have been drawn out so fine that over 30,000 of them laid side by side would measure only one inch across, and a mile of such wire would weigh only a grain, or one seven-thousandth of a pound.
Malleability.—This is considered a modification of ductility. Any metal which can be beaten out, as with a hammer, or flattened into sheets with rollers, is considered malleable. Gold possesses this property to the highest degree. It has been beaten into leaves one three-hundred-thousandth of an inch thick.
Hardness.—This is the resistance which bodies offer to being scratched by others. As an example, the diamond has the capacity to scratch all, but cannot be scratched by any other.
Alloys.—Alloys, that is a combination of two or more metals, are harder than the pure metals, and for this reason jewelry, and coins, are usually alloyed.
The resistance of a body to compression does not depend upon its hardness. Strike a diamond with a hammer and it flies to pieces, but wood does not. One is brittle and the other is tough.
The machinist can utilize this property by understanding that velocity enables a soft material[p. 82] to cut a harder one. Thus, a wrought iron disc rotating rapidly, will cut such hard substances as agate or quartz.
Resistance.—All metals offer more or less resistance to the flow of an electric current. Silver offers the least resistance, and German silver the greatest. Temperature also affects the flow. It passes more easily over a cold than a warm conductor.
Persistence.—All metals on receiving heat, will retain it for a certain length of time, and will finally cool down to the temperature of the surrounding atmosphere. Some, like aluminum, retain it for a long time; others, as iron, will give it off quickly.