Chemical compounds existing in nature, and also produced chemically, possessing the unique and valuable property of being able, when saturated with solution of sodium chloride, to take up from water passed through them such hardening elements as calcium or magnesium, and to exchange the same for salt when brought in contact therewith. Permutit Co. v. Graver Corporation, D.C.I11., 37 F.2d 385, 386.