" phosphoric acid, phosphorous acid, amidosulfonic acid, sulfuric acid, sulfurous acid, thiosulfuric acid, HSCN and hydrogen sulfide, especially acetic acid. [0125] Examples of oxides are carbon dioxide, sulfur dioxide, sulfur trioxide and phosphorus oxide, especially carbon dioxide. [0126] It is preferred to select a salt whose decomposition products are of little or no toxicity and/or of little o" . . . .