12 PHOSPHORESCENCE 



no substance surpasses in this respect sulphuret 

 of barium. 



It is nowa long time since the cobbler of Bologna,, 

 in Italy, astonished and amused his friends with a 

 peculiar substance since known as Bologna phos- 

 phorus, Bologna stone, or Solar phosphorus, which 

 shines brightly in the dark after having been placed 

 in the sunlight for some time. This substance is 

 sulphuret of barium. The cobbler prepared it by 

 heating red-hot with charcoal a piece of sulphate 

 of baryta, or Barytine, (Fig. I,) a stone which he 



Fig. 1. 



picked up in the secondary strata of the Monte 

 Paterno, where he found it in lumps of considerable 

 weight.* The German chemist, Marggraf, used to 

 prepare solar phosphorus by powdering down the 

 stone, and making it into thin cakes, with a mix- 

 ture of flour and water, before submitting it to 

 calcination. This " Bologna phosphorus" was the 

 first substance known to become phosphorescent 

 after insolation, and, consequently, it has been 



* Barytine is found abundantly in Derbyshire, Cumberland, 

 the Isle of Arran, etc. . 



