Prof. Draper on the Phosphorescence of Bodies. ' 95 



requires that one light should be sixty times as intense as the 

 other ; it follows, therefore, that at those distances the illumi- 

 nating power of the lamp is fifteen hundi-ed times as intense as 

 the illuminating effect of the spar. 



But the quantity of spar used in this experiment exposed a 

 sm'face much greater than that of the flame, — it was estimated 

 at least twice as great. This, therefore, would bring us to the 

 conclusion, that the intrinsic brilliancy of the chlorophane is not 

 y^^y^dth part of that of the lamp. 



This experiment was several times repeated. Thus it was 

 found that the lamp extinguished the shadow from the spar when 

 their relative distances were 1 : 4. The lamp at 4 was therefore 

 sixty times as luminous as the spar at 1 ; that is, their illumina- 

 ting power is as 1 : 960. But it was estimated that the surface 

 of the spar employed was 3|- times that of the flame of the lamp ; 

 so this would make the intrinsic brilliancy ■^^'^oth, a result of 

 the same order as the preceding. 



From this we conclude, that the intrinsic hriUiancy of phos- 

 phori is very small ; a fine specimen of chlorophane, at its maxi- 

 mum of brightness, yieldinr/ a light three thousand times less intense 

 than the flame of a very small oil-lamp. 



It was stated above, that these photometric experiments put a 

 new view on the whole subject ; in fact, they explain all the dif- 

 ficulties of the foregoing inquiries. How could we expect to be 

 able to measure the heat of phosphorescence ? The radiant heat 

 of the little oil-lamp here employed at such distances would re- 

 quire a very delicate thermometer to measure it. Is it likely, 

 then, that we could detect that of a source three thousand times 

 less intense ? 



I conclude, therefore, that all phosphoric bodies emit radiant 

 heat as well as light ; but that its quantity is so small, that we 

 have no means delicate enough to measure it, though the eye is 

 so sensitive that it can detect the light, the absolute intensity of 

 which has, however, hitherto been greatly over-rated. I believe 

 that the quantities of both arc of the same order ; and if this be 

 true, we should scarcely expect to discover any dilatation of the 

 glowing body, unless means much niore refined than those here 

 resorted to are employed. 



On the effects of Temperature on Phosphorescence. 



It has been already observed, that the effect of heat in pro- 

 moting the disengagement of light is an old discovery, dating 

 back at least to the times of Alliertus Magnus, who remarked it 

 in the case of a diamond j)lunged in hot water. 



It is customai-y in later works which treat systematically on 

 phosphorescence, to group the different facts under two heads : — 



