Phosphorescence 307 



Cascariolo or Casciorolo) , the cobbler of Bologna, who dabbled 

 in alchemy. He found, apparently by chance, that a mineral ob- 

 tained from Monte Paterno near Bologna, would, after calcining, 

 glow in the dark. The mineral was heavy spar, a native barium 

 sulphate, rich in sulphur. Properly prepared, it would attract the 

 " golden light of the sun " and seemed to be the appropriate ma- 

 terial to convert ignoble metals into gold, whose alchemistic symbol 

 was the sun (sol) . In later years many prominent men ^ were to 

 tread the slopes of Monte Paterno near Bologna, searching for the 

 small (size of a walnut or orange) heavy silvery stones which they 

 thought promised so much. 



Cascariolo showed his " lapis Solaris " to Scipio Bagatello (or 

 Begatello) , well known at the time in the art of gold making, and 

 also to Giovanni Antonio Magini (1555-1617) , the astronomer and 

 professor of mathematics at Bologna, who spread the news quickly 

 among their friends. Although the hope of preparing a philoso- 

 pher's stone failed, the discovery of a material which would imbibe 

 the light of day aroused the interest of learned men throughout 

 Italy. Many knew of the Bolognian stone, a name often used indis- 

 criminately for heavy spar itself or for the material prepared from 

 the stone, more properly called the Bolognian phosphorus. For 

 some time the method of preparation was kept more or less a secret. 



Galileo (1564-1642) joined in the general discussion^ of the 

 " stone " although he never wrote a special treatise on the subject. 

 He presented samples to the eminent Gulio Cesare La Galla (1576- 

 1624) , a professor of philosophy at the Collegio Romano in Rome, 

 the first to mention its properties in writing. In the book, De phe- 

 nomenis in Orbe Limae, etc., Venetis, 1612, whose title page is 

 reproduced as figure 29, La Galla made it clear that the original 

 stone did not luminesce but attained that property only after it 

 had been heated into a calx. Exposure to twilight as well as sun- 

 light made the calx appear like a glowing coal. The ability to lumi- 

 nesce was lost after a time, owing, as is now known, to the absorp- 

 tion of moisture. La Galla's explanation of the behavior was that 

 a certain amount of the fire and light substance to which it had been 

 exposed became confined in the stone and later passed out slowly. 

 Light must be absorbed, as a sponge absorbs water, and the behavior 

 was taken to indicate that lisrht is a material substance. 



^ Goethe was one of them, in 1786, ahhough he was not looking for the philosopher's 

 stone. See Winderlich (1936) . 



« According to Olschki's (1927) Galilei und seine Zeit (p. 454) , the letters are in 

 ed. naz. XI 136, 140, 371, 505, 513, 515, and XIII 339, 340 of Galileo's works. 



