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 light 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. Niepce's own words, but 

 as I condensed it from his paper for the English press in 1858. 



