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 to cut a harder one. Thus, a wrought iron disc rotating rapidly, will cut such hard substances as agate or quartz.
Title: Practical Mechanics for Boys
Author: J. S. Zerbe