On heating to redness di-hydrogen sodium orthophosphate, H₂NaPO₄, or microcosmic salt, H(NH₄)NaPO₄.4.H₂O, water, or, in the latter case, ammonia in addition, is lost, and the residue consists of sodium hexa-metaphosphate, (NaPO₃)₆. It is a glass soluble in water ; its salts are mostly gelatinous. The acid, which is probably also hexa-metaphosphoric acid, is a soluble glass, formed on igniting ortho-phosphoric acid; it yields salts like that mentioned on p. 126. Unlike the other two phosphoric acids, it coagulates a solution of white-of-egg or albumen in water. Its silver salt is white and gelatinous. Mono-metaphosphates are insoluble salts, produced by igniting together oxides, carbonates, sulphates, or nitrates with excess of phosphoric acid, and removing the excess of phosphoric acid with water. The salts of the alkalies are sparingly soluble. Metarsenates are produced in a similar manner to the hexa-metaphosphates, but, on treatment with water, they combine with water and re-form the orthoarsenates of metal and hydrogen from which they were obtained. Some pyro- and meta-thio-arsenates have been prepared.