526 M. Becquerel on the Luminous Effects 



chemical conipositioUj but also according to certain molecular 

 iiiotlitications which may be produced in substances having the 

 same chemical constitution. For instance, when phosphorescent 

 sulphuret of calcium is prepared b}' means of carbonate of lime, 

 the luminous effects which are obtained are more or less brilliant, 

 and the tints are different shades of violet. If, on the other 

 hand, this same substance be prepared by reducing the sulphate 

 of lime with carbon, the emitted light will vary in tint from an 

 orange-yellow to a feeble violet. Other minerals, such as dia- 

 mond and fluor-spar, present very great differences, not only in 

 the intensity, but also in the colour of the emitted light, and 

 that most probably in consequence of different molecular con- 

 ditions. 



In the second part of his research, relative to the action of light 

 upon phosphorescent bodies, the author first occupies himself with 

 the action upon phosphorescent substances at the ordinary tem- 

 perature, of rays of different refrangibilities. It was already known 

 that the more refrangible rays, and particularly the violet, were 

 most efficacious in giving rise to phosphorescent effects. M. 

 Becquerel has shown that the solar rays which most excite phos- 

 phorescence are still more refrangible than the violet rays, that 

 in fact they are situated partly without the spectrum. By giving 

 to each body different positions in a very concentrated spectrum, 

 and for each position examining the phosphorescence which 

 accompanies the action of radiation, the author has observed 

 that each substance, iudeed each preparation, is sensitive within 

 different limits of refrangibility. The author here enters into very 

 interesting details on the manner in which the several phospho- 

 rescent bodies above described behave under the action of I'ays 

 of different refrangibilities ; and he shows not only that phos])ho- 

 rescence is developed most actively by the action of the more 

 refrangible rays of the spectrum, but also that the action of the 

 less refrangible rays tends, on the contrary, to diminish and even 

 to destroy phos])horesceuce. 



M. Becquerel next treats of the duration of these phospho- 

 rescent effects, and shows that the time (generally very short) 

 during which phosphorescent substances can emit light in the 

 dark is extremely variable, and depends upon the sensitiveness 

 of the substances as well as u])on their temjierature. In no case 

 is there any relation between the duration of the luminous effect 

 and the intensity of the emitted light. Thus there are certain 

 minerals, such as Arragonite, Iceland spar, and glass, which 

 emit a veiy vivid light whose duration scarcely exceeds fifteen or 

 twenty seconds ; whei-eas chlorophane and certain diamonds, 

 much less luminous than Arragonite, continue to emit light for 

 upwards of an hour after insolation. The same takes place with 



