332 



SCIENTIFIC RECREATIONS. 



reproduced. The intensity of the light emitted after insolation is always 

 much less than that of the incidental light." These phenomena appear to 

 have been first observed with precious stones; then, in 1604, m calcined 

 Bologna stone, and later, in a diamond by Boyle, in 1663 ; in 1675 it was 

 noticed in Baudoin phosphorus (residuum of the calcination of nitrate of lime), 

 and more recently still in connection with other substances which we will 

 mention. The substances most powerfully influenced by the action of light 

 are sulphates of calcium and barium, sulphate of strontium, certain kinds of 



Fig. 319. Artificial flower coated with phosphorescent powder, 

 exposed to the light of magnesium wire. 



diamonds, and that variety of fluoride of calcium, which has received the 

 name of clilorophanc. 



Phosphorescent sulphate of calcium is prepared by calcining in an 

 earthenware crucible a mixture of flowers of sulphur and carbonate of lime. 

 But the preparation only succeeds with carbonate of lime of a particular 

 character. That obtained from the calcination of oyster shells produces 

 very good results. Three parts of this substance is mixed with one part of 

 flowers of sulphur, and is made red-hot in a crucible covered in from contact 

 with the air. The substance thus obtained gives, after its insolation, a 

 yellow light in the dark. The shells of oysters, however, are not always 



