Persulphuric anhydride and the corresponding acid have been known for some time. Berthelot obtained the former by subjecting a mixture of sulphurous anhydride and oxygen to the effluve électrique (as in the preparation of ozone), and a mixture of the latter with sulphuric acid, by adding the anhydride to water. He also prepared a similar mixture by the electrolysis of sulphuric acid solution in a cell where the electrodes were separated by a porous pot. Both substances he found to be very easily decomposed, spontaneously evolving oxygen. Up till now, however, the corresponding salts have not been prepared. In fact, Mendeléef, while commenting on Berthelot's results, expresses the opinion that persulphuric anhydride is not a true acid-forming oxide, but a peroxide similar to those of the metals barium, lead, &c, and that Berthelot's persulphuric acid is analogous to peroxide of hydrogen. Recently, however, I have obtained the potassium salt, and have since succeeded in preparing it in quantity.