“The form of apparatus in use varies from a portable jar, with an inlet at the top and an outlet at the bottom, to the more complex tanks for industrial and domestic purposes. A plant for domestic use might consist of a 20-gallon tank for containing the softening material and a second tank in which is prepared the salt solution for reactivating the softener. The two tanks with their valves and connections constitute the apparatus. The softener, supported by a porous plate, sieve, or layer of gravel, completely fills the first tank and the water to be treated passes through the interspaces between the granules. In some plants the water passes through a layer of marble chips before coming into contact with the softener. The apparatus may be attached temporarily to the faucet or connected permanently with the water system. A gravity system may be employed where the water pressure is lacking. “The softener is put on the market in granular form and may be purchased and used with apparatus other than that furnished by manufacturers. The granules are about 1/4 inch in diameter and permit a ready passage of the water through the interspaces. The material lasts indefinitely. “As the water passes through the apparatus, the large exposed surface of the granules entirely absorbs the calcium and magnesium, which produce hardness, making it soft and ready for immediate use. The water does not require being in contact with the softener any longer than the time taken to pass through and it emerges almost as fast as from the faucet. The softener must be reactivated after it has softened a certain amount of water. This is accomplished by filling the tank with a common salt solution which is contained in the second tank. The water supply is temporarily shut off and the salt solution allowed to fill the softening tank. After remaining in contact with the granules for a time the chemical action of the salt releases the calcium and magnesium, which are flushed out with the excess of salt solution, into the sewer. The softener thus renewed is ready for softening another supply of water. Since this renewal is a simple application of the law of mass action, an excess of the salt must be used. The renewal may be repeated indefinitely. “The amount of any particular sample of water which can be softened before renewal depends on the amount of material in the apparatus and the hardness of the water. Five gallons of the water per pound of softener would not be far from the average capacity. Where a large amount of soft water is required at one time, it may be prepared in advance and accumulated in a tank or cistern. “The cost of softening, aside from the original cost of the plant, is nominal, as the value of the salt solution is the only expense.