^ Mr. Powell on Light and Heat 



in whatever proportion the radiant matter from any source affects 

 the two bulbs, if that radiant matter be of one simple kind, the 

 only diflference on removing the screen will be, that the intensity 

 of its action on the bulbs w^U be increased; but it v/ill act on 

 each in the same proportion as before ; consequently an increase 

 of effect, or a motion in the same direction as before, must, of 

 necessity, take place. If, therefore, we perceive the reverse of 

 this take place, it is a decisive proof that when the screen is re- 

 moved, a different agent is brought into action. 



But the effect of the screen will probably interfere to too great a 

 degree to allow of this conclusion, without further precautions. 



The influence of the screen -will be reduced to nothing, if we 

 can operate at a sufficient distance. Or again, we may judge of 

 its effect by moving it nearer to the source of heat, when its cool- 

 ing influence (if any) must be diminished. In observing the effect 

 produced on the instrument during a short interval of time, 30 

 seconds for example, the effect of the screen must be of a cooling 

 nature ; and since, on the principle above stated, equality of dis- 

 tance in the bulbs is not an essential condition, we may place them 

 obliquely, (fig. 3.) care being only taken that they remain exactly 

 in the same condition when the screen is removed ; in this way 

 the difficulty is obviated. The black bulb being the worst ra- 

 diator, by placing it nearest the screen, the cooling power will 

 be displayed in the less apparent effect on this bulb ; but if, in 

 this position, an effect take place on this bulb from the transmitted 

 portion of the radiation, it will be evident that on removing the 

 screen, the effect, if due to one simple agent, ought to increase, 

 from a twofold cause, the removal of the cooling mass, and the 

 admission of a new portion of heat to the bulb. 



In various trials of this sort, I never found an increase under 

 these circumstances, and often a decrease ; that is, the action on 

 the other, or more absorptive, bulb was now increased ; or the 

 portion of the heat before intercepted has a different relation to 

 surfaces from that transmitted. The effect was observed from 

 incandescent metal, and from the flame of lamps and candles. 

 In these experiments, the effect is often very small; I have 



