15(5 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 



fects had failed, I varied the experiment as follows : In a 

 Bohemian glass tube a quantity of chlorophane in coarse 

 fragments, sufficient to occupy about three inches in 

 length of the tube, was placed. The reflecting camera 

 with its sensitive silver plate was set in a proper po- 

 sition. When everything was arranged, a spirit-lamp 

 was applied to the chlorophane, which soon emitted a 

 superb emerald light, and continued to do so for about 

 two minutes. As the light began to decline, the spar 

 splintered by decrepitation. The process went on in a 

 very satisfactory way. An oil-lamp was then placed in 

 front of the camera for five seconds. On developing, the 

 image of the lamp-flame came out, but no trace whatever 

 of the chlorophane could be detected. Thus it appears 

 that the splendid green light emitted when the spar is 

 heated is at least twenty-four times less intense than the 

 light emitted by a small oil flame. It should be remem- 

 bered that this is a measure of absolute intensity, and 

 not of illuminating power. 



But as it is known that green light is not very ef- 

 ficient in changing a sensitive surface, I tried to deter- 

 mine the intensity of the light emitted by chlorophane 

 by the optical method of Bouguer, described in the first 

 of these Memoirs, p. 39. 



The spar being heated by a current of hot air arising 

 from the flame of a spirit-lamp, the light of which was 

 carefully screened by a chimney and other contrivances 

 of sheet-iron, a comparison was made with a very small 

 oil-lamp, the flame of w 7 hich was about six tenths of an 

 inch high and the wick one sixth of an inch thick. It 

 was covered with a glass shade. 



The spar, when it began to glow, cast a reddish shad- 

 ow on the paper, which shadow was extinguished when 

 at its maximum by the lamp at about 25 inches, the 

 spar being at 5 inches. 



