90 Prof. Draper on the Phosphorescence of Bodies. 



When the spark was made to pass, there was a great movement 

 in the index-tube, as in the instrument known as Kinnersley's 

 electrometer ; but the liquid innnediately returned to within a 

 short distance of its first place ; then a slow dilatation occurred, 

 as though the air was gradually warming up. Thus in one ex- 

 periment the liquid stood at 24° ; after the explosion it returned 

 to 26°, and then there was a gradual dilatation to 32°. 



To eliminate the various disturbing causes in this experiment, 

 it was repeated many times, the spar being alternately introduced 

 into the glass tube and removed. It appeared that whenever 

 the spar was present, the gradual dilatation alluded to took place ; 

 but when the spar was not in the tube, instead of a dilatation 

 there was a gradual contraction, imtil the index liquid recovered 

 its original position. 



From this it appears, that with the evolution of light there is a 

 feeble extrication of heat. 



The quantities of heat thus liberated are so small, and the 

 causes of error so numerous, that I endeavoured by other me- 

 thods to obtain more reliable results. Thus, I attempted to 

 determine the surface temperatm-e of a fiat piece of chlorophane, 

 while phosphorescing, by means of the thermometric multiplier. 

 The pile was placed in a vertical position, and the spar having 

 been attached to a strip of wood which sei-ved as a handle, in- 

 tense phosphorescence was communicated by a Leyden spark, 

 and the fiat and shining surface instantly put on the ixpper 

 face of the pile. But there was no movement of the astatic 

 needles. 



Then, taking the stone by its handle, it was touched with the 

 tip of the finger for one second, and quickly placed on the pile. 

 A prompt movement of the needles, amounting to four degrees, 

 ensued. These experiments were repeatedly tried, and the re- 

 sults were uniformly the same. 



On considering these results, it appears that, as the tempera- 

 ture of the air near the multiplier in one of the experiments was 

 53°, and the estimated temperature of the skin 98°, the amount 

 of heat which the stone received from the touch of the finger 

 must have been very small. I made a comparative trial, by 

 touching the bulb of a thermometer for the same space of time 

 in the same way, and found that there was a rise of about 1^°. 

 But the conductibility of quicksilver is much greater than that 

 of chlorophane. 



It is to be inferred, therefore, that the quantity of heat set free 

 during phos])horescence is veiy small, and that the surface of 

 the chlorophane does not change its temperature by one-third of 

 a degi-ee ; for had it done so, the multiplier would have instantly 

 detected it. 



