​HYPOSULPHITE OF SODA, the name originally given to the substance known in chemistry as sodium thiosulphate, Na₂S₂O₃; the earlier name is still commonly used, especially by photographers, who employ this chemical as a fixer. In systematic chemistry, sodium hyposulphite is a salt of hyposulphurous acid, to which Schutzenberger gave the formula H₂SO₂, but which Bernthsen showed to be H₂S₂O₄. (See Sulphur.)