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. Niépce de St. Victor, as stated in Chapter VII. 
(Part I.). 
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 1s 
yet in its infancy, and that the greatest philoso- 
phers of the present day know less of it than of 
any other physical phenomenon. 
