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CHAPTER I. 

 PHOSPHORESCENCE AFTER INSOLATION. 



SEVERAL substances manifest the strange property 

 of emitting light when they are placed in darkness, 

 after having been exposed for some time to the 

 direct rays of the sun. In some cases a very short 

 exposure to sunlight is sufficient to excite the 

 manifestation of this remarkable property, and in 

 others the direct rays of the sun are not necessary : 

 it suffices that the substance experimented upon 

 be exposed to the dull light of a cloudy day. To 

 this phenomenon the denomination of Phosphores- 

 cence after insolation has been given. 



' The substances which possess this property in 

 the highest degree are the Bologna stone, or solar 

 phosphorus, certain varieties of fluor-spar and 

 carbonate of lime, some fossils, calcined shells or 

 pearls, phosphate of lime, arseniate of lime, etc. 

 Many diamonds shine with brilliancy in the dark 

 if they have previously undergone an exposure of 

 some seconds' duration only to solar light. But 



