214 Mr. Powell on Lt^ht and Heat, 



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Tipper passing in contact with the lower, must not be used without 

 its case, and should also be screened by glass, for if it be exposed, 

 the heating power of the light no longer acts upon it alone, but 

 the effect is increased from other causes. If the bulb be painted 

 black, this increase is in part owing to the greater absorptive 

 power of the coated, than that of the plain bulb, for the simple heat 

 admitted to it by the removal of the case and screen. But partly 

 also it is owing to another cause. The stem which passes in con- 

 tact with the lower bulb being of thicker glass, is longer in ac- 

 quiring heat than the lower bulb is ; and therefore will initially 

 cool it, and thus increase the apparent effect on the other bulb. 

 Such an increase is owing-to the presence of simple heat, and could 

 never be produced by light. 



(2) When the instrument employed is of the " stationary" kind, 

 that is, having its bulbs at equal heights, and (as in the one I used) 

 having one bulb blown of black enamel, there are other consider- 

 ations to be attended to. In fact, I have found that this instru- 

 ment, when exposed without case or screen, not only exhibits no 

 effect upon the black bulb, but shews a considerable depression on 

 the side of the plain bulb ; this only continues for about two mi- 

 nutes, when the other begins to be affected. The same thing takes 

 place, perhaps in a less degree, if the case be used, and become 

 considerably heated ; the saiAe also occurs if the instrument be 

 used without its case, but screened by a piece of glass, the glass 

 becoming heated. When both the case and a glass screen are 

 employed, this effect is not produced, but the instrument remains 

 stationary, or nearly so ; sometimes a trifling effect seems to be 

 produced on the black bulb. But if a second screen be used also, 

 I have always found an effect of about two degrees, or rather 

 more, in one minute on the black bulb. The outer screen becomes 

 hot, the second very little so, and the case remains quite cool ; the 

 screens were from 1-lOth to l-8th inch in thickness. 



(3.) It is impossible to ascribe the effect through the screens to 

 any thing but the heating power of light ; but it is possible that it 

 is less than what ought to be the case, if we suppose the opposite 



