produced by the action of Light upon Bodies. 531 



regains its primitive action. Thus, between —20° and +200° 

 the sulphuret of strontium of violet luminosity (prepared 

 K'nnoN ^^^^^^^^ of sulphur on strontia at a temperature above 

 500 ) presents temporarily and successively almost all the 

 prismatic colours, with the exception of red ; that is to say, it 

 exhibits the same effects as those occasioned by the different pre- 

 parations of sulphuret of strontium when maintained at the ordi- 

 nary temperature. These temporary changes in the intensity and 

 refrangibility of the light emitted by a body, due to the influ- 

 ence ot heat, support our third conclusion, by showing that the 

 physical condition of a body, and not merely its chemical com- 

 position, produces variations in the effects of phosphorescence. 



10. In general, as already mentioned in the first conclusion, 

 the refrangibijity of the light emitted by phosphorescence is less 

 than that of the exciting rays— or at least the wave-lengths of the 

 rays which bodies emit after being subjected to the action of ra- 

 diation, are greater than those of the exciting rays ; nevertheless 

 there are cases (the sulphuret of calcium, of indigo-blue lumino- 

 sity, IS one) where the wave-lengths are the same; under these cir- 

 cumstances the substance vibrates in unison with the exciting rays. 

 K V" u^^ *"'"^ necessary for the phosphorescent excitation of 

 bodies by luminous radiation is extremely short, since an electric 

 spark whose duration is less than ToTiWrth of a second suffices 

 to occasion the phsenomenon of phosphorescence. Nevertheless, 

 to obtain the maximum effect at a given temperature and under 

 the influence of a given luminous intensity, it is probable that 

 the time of insolation depends upon the intensity of the exciting 

 rays and upon the degree of sensitiveness of the substance. 



12. The rays which emanate from a phosphorescent body pre- 

 viously subjected to simple insolation have not sufficient inten- 

 sity to aftect thermometers, nor has it hitherto been possible to 

 l)roduce auy'chemical action by their influence. 



13. Many bodies, such as glasses and certain compounds of 

 uranium, owe their fluorescence entirely to the persistence of the 

 luminous impression for a very short time, which does not exceed 

 a few hundredths of a second as measured by the phosphoroscope ; 

 the intensity of the emitted light is then very brilliant. Pos- 

 sibly other fluorescent bodies, and particularly organic substances, 

 present analogous effects ; but if so, the duration of the persist- 

 ence of the luminous influence must be much shorter (for, with 

 the phosphoroscopes I have hitherto employed, I have not been 

 able to render the same perceptible), unless, indeed, in certain 

 bodies the emission of light only takes place during radiation. 

 At all events it is probable that phosphorescence and fluorescence 

 differ from one another only in the time during wlych a lumiuous 

 impression is preserved in bodies. 



3M2 



