PHOSPHORESCENCE. 75 
board into a metallic tube, so that it lines the 
latter. I close the tube, and after a very long 
period of time I prove, by opening it, that the 
cardboard acts upon salts of silver as perfectly as 
it did the day it was prepared. At the ordinary 
temperature, this action becomes manifest only 
after a period of twenty-four hours; but if, after 
opening the tube, a few drops of water are thrown 
into it, and it be then closed again, and heated to 
forty or fifty degrees (centigrade), on re-opening 
the tube, and applying its orifice to a photogra- 
phic paper, an impression is produced upon the 
latter in less than five minutes. This experiment 
only succeeds once, as if the photographic paper 
had absorbed all the ight out of the tube; and to 
produce a second impression, the cardboard must 
be again exposed to light.’’* 
This shows that heat and chemical action have 
an influence in these phenomena, and we know 
that this is very generally the case in phenomena 
of phosphorescence. 
Mr. Draper, in his ‘Human Physiology,’ p. 288, 
describes an experiment which is closely allied to 
the above :—If a sheet of paper, upon which a key 
has been laid, be exposed for some minutes to the 
sunshine, and then instantaneously viewed in the 
dark, the key being removed, a fading spectre of 
* This passage is not entirely in M. Niépce’s own words, but 
as I condensed it from his paper for the English press in 1858. 
