h which has empirical formula ##STR2## wherein X is a chalcogen, each of R1 and R2 is hydrogen, NR3 R4 or NR5, at least one of R1 and R2 being other than hydrogen, each of R3 and R4 is hydrogen or a monovalent organic radical and R5 is a divalent organic radical, in which a molar ratio of the chalcogen-containing compound to the acid is about 1 or more, and the molar ratio of water to the combinat