The term hard is applied to those waters which form a heavy curd with soap. The hardness is due to the solution of excessive quantities of the salts of magnesia and other similar mineral substances. Since these salts are found held in solution by the carbon dioxid which the water carries, the boiling of hard water will precipitate part of the mineral matters thus held. The original hardness of such water is called the total hardness, and the hardness which remains after the precipitation of the bodies above mentioned the permanent hardness. The hardness of water is more important from its technical use perhaps than from its use as a beverage. Hard waters, however, on account of the large quantities of mineral contained therein, are not suitable for permanent consumption. Technically they are objectionable, especially for use in boilers because of the coating of the boilers produced by the precipitation of the mineral matters, often in very firm layers, upon the surfaces of the boiler. This precipitation is often seen in vessels used in the kitchen where hard water is employed. Various precipitants have been used in order to diminish the hardness of water and make it suitable for beverages and for technical purposes.