1825.] Mr. Powell on Terrestrial Light ami Heat. 361 



be both alike absorptive of simple radiant heat. This was not 

 exactly the case in the following experiments ; one bulb beinf 

 coated with Indian ink, so that if the radiation were not sufficient 

 to counterbalance the effect, the value given to (/) is too great 

 when the instrument was used without its case ; but the differ- 

 ence was probably very trifling as will appear by a comparison 

 made in some of the experiments. The annexed sketch will, 

 perhaps, assist in showing the nature of the effects : it requires 

 no explanation. 



-<y 



Light- 

 Heat.. . 



A. 

 B. 



Hot body. 



Differ, thermometer. 



C. S 



creen. 



This is precisely the same method as I formerly adopted for 

 endeavouring to detect any sensible degree of non-transmissible 

 heat in the sun's rays. 



(5.) In the instance of the sun then, the heating power of 

 light constitutes the total effect. In the instance of luminous 

 terrestrial sources, we recognise the joint action of the two 

 radiations ; and in non-luminous hot bodies only that of heat. 

 In different instances of luminous bodies, these two causes 

 operate in different proportions so as in some to approach the 

 first, and in others the last of these descriptions ; and if so, what 

 are the distinctive circumstances with which such variation is 

 accompanied ? 



(6.) In addition lo the inference before made from De la 

 Tloche's experiments, it seems well established that {cateris 

 pari/jKs) the light emitted from flame increases with the com- 

 pleteness of the combustion. Tiius Count Rumford (Phil. 

 Essays, i. 304) found that with equal quantities of oil, the light 

 of an argand lamp was to that of a common lamp us 100 to 85. 

 I was desirous of comparing such a ratio with the correspond- 

 ing one of the effects of simple heat; and the following are a 

 few results obtained bv the method just described, with the 

 name of an argand lamp (the diameter of whose cylindrical wick 

 was 0"75 inch), by increasing the flame. The first experiment 

 was made for the sake of comparison, in order to estimate the 

 effect of the glass chimney : the instrimient was one having the 

 bulbs at the same height; the sentient bulb coated with Indian 

 ink. 



