326 PHOSPHORESCENCE. [SECT. xxvm. 



friction ; for Sir David Brewster observed that bodies de- 

 prived of the faculty of emitting the one are still capable 

 of giving out the other. Among the bodies which generally 

 become phosphorescent when exposed to heat, there are 

 some specimens which do not possess this property ; where- 

 fore phosphorescence cannot be regarded as an essential 

 character of the minerals possessing it. Sulphuret of cal- 

 cium, known as Canton's phosphorus, and the sulphuret of 

 barium, or Bologna stone, possess the phosphorescent pro- 

 perty in an eminent degree ; and M. Edmond Becquerel has 

 shown that, on these substances, a very remarkable phos- 

 phorescent effect is produced by the action of the different 

 rays of the solar spectrum. In former times Beccaria stated 

 that the violet ray was the most energetic, and the red ray 

 the least so, in exciting phosphoric light. M. Becquerel 

 has shown that two luminous bands, separated by a dark 

 one, are excited by the solar spectrum on paper covered 

 with a solution of gum-arabic, and strewed with powdered 

 sulphuret of calcium. One of the luminous bands occupies 

 the space under the least refrangible violet rays, and the 

 other that beyond the lavender rays, so that the dark band 

 lies on the part under the extreme violet and lavender rays. 

 When the action of the spectral light is continued, the whole 

 surface beyond the least refrangible violet shines, the lu- 

 minous 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 

 calcium 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 re- 

 mains luminous except the part from the least refrangible 

 violet to the extreme red, on which space the light is extin- 

 guished, 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, 



