n. The state or quality of being hard, in any of the senses of that word; solidity; density; difficulty of comprehension, accomplishment, control, or endurance; obduracy; harshness; severity; inclemency; adversity; roughness; uncomeliness; want of sensibility.n. Specifically That quality in fountain-water which is imparted by the presence in excess of earthy salts, especially calcium sulphate.n. In medicine, that quality of the pulse which is due to tension of the artery, which in this condition does not readily yield to the pressure of the finger.n. In art and music, harshness or coldness of execution; unsympathetic treatment, as of a tone or the details of a picture; want of feeling in performance.n. In mineralogy, the comparative capacity of a substance to scratch another or be scratched by another; the quality of bodies which enables them to resist abrasion of their surfaces.n. Water, as found in nature, containing salts of lime or magnesia or both of these in considerable quantity, is said to be hard; it curdles or precipitates soap bv forming insoluble lime or magnesia salts of the fatty acids. Any lime or magnesia present in the condition of carbonate is held in solution by carbonic acid, and if this latter is driven off as carbon-dioxid gas by boiling the water, the earthy carbonates are precipitated, so that the water is to this extent softened. The part of the original hardness which is thus removable by boiling is called temporary hardness. The part due to calcium or magnesium in the condition of chlorid or sulphate is not thus removable, and is called permanent hardness. The sum of the temporary and permanent hardness constitutes the total hardness. Hardness is frequently stated in degrees, each degree representing hardness equivalent to that caused by 1 grain of calcium carbonate in 1 imperial gallon of water; or, now more commonly, 1 part of calcium carbonate in 1,000,000 parts of water.
