In the country about Rome there is a very hard ſtone, which is hewn out of the quarry juſt like other ſtones for building: this ſtone yields a great deal of Alum. In order to extract it, the ſtones are calcined for twelve or fourteen hours; after which they are expoſed to the air in heaps, and carefully watered three or four times a day for forty days together. In that time they begin to effloreſce, and to throw out a reddiſh matter on their ſurface.