Summary: ALTHOUGH the hardness of a substance is not a physical constant, and cannot be expressed as a function of known physical constants, the idea of hardness is a familiar one, and empirical determinations of hardness play a great part in the study of materials. Methods depending on scratching now take only a subordinate place, and ‘resistance to indentation’ is the most generally accepted definition of hardness. The Brinell test, in which a hard ball is pressed into the object under a known load, forms the basis of most hardness testing, but the introduction of the diamond pyramid, as in the Vickers test, in place of a steel ball, has given greater precision to the test.