76 PHOSPHORESCENCE. 
the key will be visible. Let this paper be put 
aside for many months where nothing can disturb 
it, and then in darkness be laid on a plate of hot 
metal, the spectre of the key will again appear. 
In the cases of bodies more highly phospho- 
rescent than paper, the spectres of many different 
objects which may have been laid on in succession 
will, on warming, emerge in their proper order. 
