216 CONCLUSION. 



under different aspects, because everything depends on the nature of the receiving sur- 

 face ; that it is the same principle which affects the eye as light, decomposes chloride 

 of silver as a tithonic ray, and makes sulphuret of lime shine as a phosphorogenic ray ; 

 that the difference is not in the radiant principle, but in the surface on which it is re- 

 ceived ? To go no farther in a discussion which has already extended this chapter too 

 much, if the agent is the same in all cases, and the difference perceived is due to the 

 -receiving surface, how is it that a ray of light which has passed through a piece of 

 transparent glass can no longer excite phosphorescence in the sulphuret of lime 1 Can 

 we escape the conclusion that the ray has had something removed from it, or has had 

 some modification impressed on it, or, in short, that something invisible to the eye has 

 been taken away 1 



997. To my mind these considerations are conclusive, and I therefore regard the 

 tithonic rays as constituting a fourth imponderable, and the phosphorogenic rays as a 

 fifth. 



THE END. 



