530 M. Becqiierel on the Luminous Effects 



ducts of the reaction of sulphur upon caustic strontia, and upon 

 lime. 



By following the indications contained in this research, phos- 

 phorescent substances may be obtained which, after solar action, 

 emit much more vivid light than any before observed. 



5. The cause which produces the phaenomenon of phosphor- 

 escence by the insolation of a substance, is probably different from 

 that whereon its crystalline state depends, and perhaps is con- 

 nected with its density, or with some peculiar physical property 

 not yet specified. Under some circumstances the power which 

 such a substance possesses of giving an emission of light of a 

 certain tint is preserved by some of its combinations. 



6. Phosphorescent sulphuret of strontium, prepared so as to 

 emit light of one or the other tint, presents by diffusion in day- 

 light a tint analogous to, though much more feeble than, that of 

 the light emitted by phosphorescence in obscurity. This effect 

 api^ears to indicate a disposition of molecules in the body capable 

 of producing a determinate luminous effect, either through dif- 

 fusion or through phosphorescence, that is, through its own 

 vibrations. An action of a similar kind is observed in the salts 

 of uranium. 



7. There is no relation whatever between the duration of the 

 light emitted by bodies after insolation, the intensity of the same, 

 and its refrangibility ; for instance, a body may emit a feeble 

 light for a long time, as in the cases of diamond and chlorophane, 

 or a very vivid light for a very short time, as in the cases of Iceland 

 spar, glass, nitrate of uranium, &c. 



8. One and the same body may emit rays of very different 

 colours, according to the time which intervenes between the ac- 

 tion of light and the moment of observation. This last result 

 shows that vibrations of different velocities are preserved for un- 

 equal times in different bodies : sometimes it is the vibrations 

 corresponding to the less refrangible rays which continue longest, 

 as in bisulphate of quinine, double cyanide of potassium and 

 platinum, diamond &c. ; sometimes, on the contrary, those cor- 

 responding to the more refrangible rays are most durable, as in 

 calcareous and Iceland spar. 



On the other hand, one and the same body may emit vibra- 

 tions of the same velocity, but having vmequal dm-atious, and that 

 according to the action of diffei'ent parts of the luminous spec- 

 trum. Such is the effect produced upon most of the sulphurets 

 of the alkaline earths. 



9. When a phosphorescent substance is maintained at a more 

 or less elevated temperature and presented to the action of lumi- 

 nous radiation, it may acquire the capability of emitting rays of a 

 definite colour ; but on returning to the ordinary temperature it 



