200 PRACTICAL CONSIDERATIONS. 



Again, no stone resembles the diamond so 

 closely as white topaz, which is often sold as dia- 

 mond. But the phosphorescent properties of the 

 latter furnish us with a ready means of detecting 

 the one from the other. 



The reproduction of engravings, etc., by phos- 

 phorescence has been achieved by my friend 

 M. Niepce de St. Victor, as stated in Chapter VII. 

 (Parti.). 



As phenomena of phosphorescence are more 

 studied, they will doubtless lead to new views 

 upon the molecular constitution of bodies, which 

 cannot fail to be attended with practical results. 



Phosphorescence will certainly be applied to 

 many other useful purposes as it becomes better 

 known. We must remember, however, that it is 

 yet in its infancy, and that the greatest philoso- 

 phers of the present day know less of it than of 

 any other physical phenomenon. 



