1*0 Mr. Powell on Light and Heat 



Copper incandescent i± 



g.4 



2d exp. 



2.8 

 Lamp, no chimney . =: — 



with chimney . == 



^ 0.9 



In all the ahove instances, the value given to (/) should be 

 Slightly diminished fof the heating effect of the screen. Also, the 

 last ratio is of necessity inaccurate, because the isimple heat Was 

 radiated from the glass chimney, and is probably greater than 

 that which would be radiated directly from the flame. I'his ratio 

 ought therefore to be increased. But even without this correc- 

 tion, it will hence be evident that in this series of luminous hot 

 todies, the heating power of light increases in a greater ratio 

 than the simple radiant heat. And this increase of ratio corre- 

 sponds to the degree of incandescence in the metal, and the com- 

 pleteness of combustion in the flame. 



4. In order to prosecute this inquiry, I made use of the fol- 

 lowing application of the differential thermometer': if both the 

 t)ulbs present vitreous surfaces, or are equally absorptive, tlie 

 "black bulb being affected by the light from a luminous hot body, 

 and both bulbs being nearly equally aflfected by its simple heat, 

 by intei-posing a small glass screen between the source of heat 

 and the plain bulb, the effect of both is exhibited on the black 

 bulb, or (l+h). Observing in the usual way we have the effect 

 6f the light , or (Z). Hence we get (K) and the ratio of the 



Tills atrangemeiit i§ represented in Figs. 4 and 5. The in- 

 Huehce of the tWo heating agents distinguished by the differently- 

 dotted lines appears as in the former cases. Perhaps the nume- 

 rical results obtained by such a mode of experimenting may not 

 be susceptible of a great degree of precision, but for the purpose 

 of ascertaining whether there was an increase of ratio or not, this 



