296 PHOSPHORESCENCE. SECT. XXVIII. 



horizon. In the night the sea became nearly as luminous as 

 before, but on the fifth night the appearance entirely ceased. 

 Captain Bonnycastle did not think it proceeded from animalcule, 

 but imagined it might be some compound of phosphorus, sud- 

 denly evplved and disposed over the surface of the sea. It 

 had probably been that peculiar form of electricity known as the 

 glow discharge, of which the author once saw a very remarkable 

 instance. 



M. E. Becquerel assures us that almost all substances are 

 phosphorescent after being exposed to the sun if instantly with- 

 drawn into darkness, and that it depends upon the arrangement 

 pf the particles and not upon chemical action. The salts of 

 uranium give the same kind of phosphorescent light as that pro- 

 duced by the violet rays of the solar spectrum. A solution of the 

 bisulphate of quinine emits a yellow phosphorescent light, whereas 

 the fluorescent light of that liquid is blue. The colours of these 

 two kinds of light are generally complementary to one another. 



Phosphorescence is probably more or less concerned in some, at 

 least, of a series of very curious experiments made by M. Niepce 

 de Saint-Victor, on what he calls the saturation of substances with 

 light. It has long been known that, if a person in an intensely 

 dark room should expose his arm to the sun through a hole in a 

 window-shutter, it will shine on being drawn into the darkness. 

 Now, M. de Saint-Victor found that if an engraving be exposed 

 for a certain time to the sun, and instantly brought into darkness, 

 it will make a photographic impression on a collodion or argen- 

 tine surface, and that anything written or drawn with tartaric 

 acid, or a solution of the salts of uranium, in large characters, is 

 reproduced even at a small distance from a sensitive surface. It 

 may be presumed that the light communicates its vibrations to 

 the surfap.es exposed to it with sufficient force to enable them to 

 disturb the unstable equilibrium of such sensitive substances as 

 collodion or the argentine salts. M. de Saint-Victor has shown 

 that tartaric acid, which is readily impressed by sunlight, is 

 neither fluorescent nor phosphorescent, whence he concludes that 

 his experiments are independent of both of these modes of action. 

 Uranium appears to have very peculiar properties : its salts are 

 strongly luminous when exposed to the sun ; they are very fluo- 

 rescent ; and the crystallized azitote of uranium becomes phos- 

 phorescent by percussion. 



