SJBCT. XXVIII. PHOSPHORESCENCE. 287 



the least refrangible violet shines, the luminous bands 

 already mentioned brightest, but all the space from the 

 least refrangible violet to the extreme red remains dark. 

 If the surface prepared with either the sulphuret of cal- 

 cium or the Bologna stone be exposed to the sun's light 

 for a short time it becomes luminous all over, but when 

 in this state a solar spectrum is thrown upon it, the 

 whole remains luminous except the part from the least 

 refrangible violet to the extreme red, on which space 

 the light is extinguished ; and when the temperature of 

 this surface is raised by a lamp, the bright parts become 

 more luminous and the dark parts remain dark. Glass 

 stained by the protoxide of copper, which transmits only 

 the red and orange rays together with the chemical rays 

 that accompany them, has ^he same effect with the less 

 refrangible part of the spectrum ; hence there can be no 

 doubt that the most refrangible and obscure rays of the 

 spectrum excite phosphorescence, while all the less re- 

 frangible rays of light and heat extinguish it. It appears 

 from the experiments of MM. Biot and Becquerel that 

 electrical disturbance produces these phosphorescent 

 effects. There is thus a mysterious connection between 

 the most refrangible rays and electricity, which the ex- 

 periments of iVI. E. Becquerel confirm, showing that 

 electricity is developed during chemical action by the 

 violet rays, that it is very feebly developed by the blue 

 and indigo, but that none is excited by the less refrangi- 

 ble part of the spectrum. 



Paper prepared with the sulphuret of barium when 

 under the solar spectrum shows only one space of max- 

 imum luminous intensity, and the destroying rays are 

 the same as in sulphuret of calcium. 



Thus the obscure rays beyond the extreme violet 

 possess the property of producing light, while the lumi- 

 nous rays have the power of extinguishing it. 



The phosphoric spectrum has inactive lines which 

 coincide with those in the luminous and chemical spec- 

 tra at least as far as it extends, but in order to be seen, 

 the spectrum must be received for a few seconds upon 

 the prepared surface through, an aperture in a dark 

 room, then the aperture must be closed, and the tem- 

 perature of the surface raised two or three hundred 



