MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 



A wonderful experiment, truly. Shadows, then, are 

 not such fleeting, such fugitive things as poets say. 

 They may bury themselves in stony substances, and be 

 made to come forth at our pleasure. 



The persistence of such surface phantoms may be strik- 

 ingly illustrated by a simple experiment in which light 

 is not concerned. If on a cold polished metal, as a new 

 razor, an object such as a small coin be laid, and the 

 metal be then breathed upon, and when the moisture 

 has had time to disappear, the coin be thrown off, though 

 now the most critical inspection of the polished surface 

 can discover no trace of any form, if we breathe once 

 more upon it a spectral image of the coin comes plainly 

 into view. And this may be done again and again. 

 Nay, more, if the razor be put carefully aside where noth- 

 ing can deteriorate its surface, and be so kept for many 

 mouths, on breathing again upon it the shadowy form 

 emerges.) 



Early in the last century two hypotheses were intro- 

 duced for the explanation of the various cases of phos- 

 phorescence : 



1. That phosphorescent bodies act like sponges to 

 light, absorbing it, and retaining it by so feeble a power 

 that very trivial causes suffice for its extrication. This 

 was the view of Lemery, and was published in 1709. 



2. That phosphorescence arises from an actual combus- 

 tion taking place in the sulphureous parts of the glow- 

 ing body. It is to be remembered that sulphur figured 

 largely in the chemistry of those days. This was the 

 hypothesis of Du Fay. 



To this celebrated electrician we owe a very able in- 

 vestigation of the phosphorescence of various bodies, and 

 especially of the diamond. He recognized the fact, over- 

 looked by Boyle, that the gem must first be exposed to 



