Hard"ness, n. [AS. heardness.]
1.
The quality or state of being hard, literally or figuratively.
The habit of authority also had given his manners some peremptory hardness.
Sir W. Scott.
2. Min.
The cohesion of the particles on the surface of a body, determined by its capacity to scratch another, or be itself scratched;-measured among minerals on a scale of which diamond and talc form the extremes.
3. Chem.
The peculiar quality exhibited by water which has mineral salts dissolved in it. Such water forms an insoluble compound with soap, and is hence unfit for washing purposes.
⇒ This quality is caused by the presence of calcium carbonate, causing temporary hardness which can be removed by boiling, or by calcium sulphate, causing permanent hardness which can not be so removed, but may be improved by the addition of sodium carbonate.
© Webster 1913.