138 Experimental Philosophy. [Lecture 10. 



explained hereafter, when we treat of lenses, 

 and of mirrors. But I do not wish to pass over 

 any thing that I mention, without an attempt to 

 render it clear to your comprehension. I men- 

 tioned the solar phosphor i , of which it is pro- 

 bable that very few of you have heard before. 

 They are certain substances which, when ex- 

 posed for a little time to the strong rays of the 

 sun, are found to imbibe a large quantity of light, 

 so that they will shine, or appear luminous, if 

 immediately carried into a dark place. The 

 most remarkable of these is the Bolognian phos- 

 phorus. It was accidentally discovered by a 

 shoemaker of Bologna. This man had collected 

 together some stones of a shining appearance at 

 the bottom of Mount Peterus, and being in 

 quest of some chemical secret (probably the 

 philosopher's stone, which was to turn every 

 thing into gold), he put them into a crucible to 

 calcine them, or reduce them to the state of a 

 cinder. Having taken them out of the crucible, 

 they were exposed to the light while he was 

 examining them, and afterwards he happened to 

 carry them into a dark place, probably to throw 

 them away; when, to his utter surprise, he 

 observed that they possessed a self-illuminating 

 power. Baldwin, of Misnia, another chemist, 

 observed some time after, that chalk, dissolved 

 in aqua fortis (after the aqua fortis had been 

 evaporated by heat, and the matter reduced to a 

 perfectly dry state), exactly resembled the Bo- 



