KNOBLAUCH ON RADIANT HEAT. 201 



a red, yellow and white heat over the chimney of a Berzelius's 

 lamp. The visible portion of the flame of the alcohol was never 

 allowed to rise above the metallic cylinder of the lamp, and the 

 thermal pile was protected from its rays by polished screens of 

 tinned iron. 



Experiment showed that when the direct radiation of heat 

 from each of the sources mentioned thrown upon the thermal pile 

 had deflected the needle to 35°, on transmission through colour- 

 less glass the heat from platinum at dark redness produced a 

 deflection of 10°'5 ; at a red heat, of 1 7°'25 ; at a yellow heat, of 

 17°'25; and at a partial white heat, of 21°-12. Thus the rays 

 from platinum at a red and yellow heat, having a great diff"erence 

 in temperature, are transmitted by colourless glass in exactly the 

 same proportion. When the glass was replaced by the plate of 

 alum Ave have so frequently mentioned, a deflection of 10°*2 in 

 the needle was produced by the platinum at dark redness; of 11°*4 

 for platinum at a red heat; of 9 •! for that at a yellow heat; 

 and of 12°'4 for that at a partial white heat. Thus the heat of 

 the platinum at a yellow heat passes through the plate of alum 

 to a less extent than that of the platinum at a red heat, nay 

 even than that of platinum at a dark red heat, notwithstanding 

 its far higher temperature. 



The same applies to gypsum. The heat of platinum at dark 

 redness passes most imperfectly through mica ; that of platinum 

 at a red heat comparatively better; that at a yellow heat still 

 better; and that at a white heat best of all. Hence we find 

 every possible case, independent of the temperature of the 

 source of heat. The following table contains the observations 

 on this point : — 



p2 



