1 



1 824.] Mr. Powell on Terrestrial Light and Heal . 1 85 



acquired heat so as to fall upon the second reflector; as will be 

 evident by inspecting the subjoined diagram, which represents 

 the arrangement of De la Roche's apparatus according to the 

 verbal description. 



It is here evident that the portions 

 of the screen which fall without the 

 area nf the rays will not radiate 

 their heat so as to produce much 

 effect on the focal bulb t, and the 

 parts b b of the blackened side will 

 give out their heat more rapidly, and 

 consequently abstract more from the other parts on the side away 

 from the thermometer than when the glass was plain. And the 

 same will be the case to a less extent even within the area of 

 the rays, since the central point of the screen will be the most 

 heated from the additional direct action of the hot body t. 



With the plain glass there was neither so great an excess of 

 heat (from its less absorptive texture), nor such a tendency to 

 radiate it on one side rather than the other. 



(7.) If the explanation here attempted be considered satis- 

 factory, it will then be obvious that we are not obliged to 

 infer from De la Roche's experiments that any simple radiant 

 heat was transmitted directly through the glass. But in the 

 subsequent cases, where the radiating body was raised to lumi- 

 nosity, it is evident that there was a much greater effect pro- 

 duced with the glass screen than can be reasonably accounted 

 for by any secondary radiation; and the magnitude of this, 

 compared with the total direct effect, obviously bears a close 

 relation to the degree of luminosity. 



(8.) In former, as well as recent times, several experimenters 

 have noticed the fact that the effect of luminous bodies on a 

 blackened thermometer is concentrated with the light in the 

 focus of a lens, and that the glass itself does not become heated. 

 See Mr. Brande's paper on Combustion, &c. Phil. Trans. 1820, 

 Part I. sect. 2.) 



Here then is a decisive proof that in these cases it is not a 

 mere secondary radiation of the heat acquired by the glass, but 

 an actual transmission. 



(9.) From the labours of the different and distinguished expe- 

 rimenters who have examined this subject, we may, with cer- 

 tainty, learn thus much, that independently of the heat acquired 

 and radiated again by the glass screen, there is, in the case of 

 luminous bodies, a certain portion of the total heating effect 

 actually transmitted. 



(10.) What then, we have now to inquire, is the nature of 

 this transmissible effect? Is it merely simple radiant heat to 

 which the great elevation of temperature communicates proper- 

 ties which at lower temperatures it does not possess >. If so, 



