Bologna Stone, or BONONIAN STONE. In the end of the 16th or beginning of the 17th century, Vincent Casciorolo, a shoemaker of Bologna, made the very remarkable discovery that the mineral now known as heavy spar (barium sulphate), which is found near Bologna (as it is in a great many other places), when reduced to a fine powder mixed with gum, dried, and strongly heated in a covered crucible, is converted into a substance having the property of shining in the dark. Casciorolo mentioned his discovery (1602) to the alchemist Scipio Begatello and the mathematician Maginus, and the latter made the substance (first called by its discoverer 'capis solaris,' but soon from the place where it was prepared, 'the Bologna, or Bononian Stone') famous by the specimens which he sent about. Peter Potier (or Poteras), a French chemist resident in Bologna, first published (1622) a recipe for making it.
The substance is essentially sulphide of barium, but its phosphorescent character depends very much on the way in which it is prepared, and many processes have been described. It shines in the dark only if it has previously been shone on, seeming slowly to give out the light it has absorbed. The now well-known 'luminous paint' is made up of this or of other similar and similarly prepared sulphides. See PHOSPHORESCENCE.