194 KNOBLAUCH ON RADIANT HEAT. 



comparatively better than that of red-hot platinum, and the heat 

 of the hydrogen flame more freely than that of the three other 

 sources. 



On this point experiment has decided as follows : — When the 

 red-hot platinum had emitted rays upon the above-described 

 thermal pile to such an extent that the needle when it was com- 

 bined with the multiplier deviated to 20°, it went back to 12° 

 when a plate of colourless glass 1*3 millim. in thickness was in- 

 troduced between the source of heat mentioned and the thermal 

 pile. These 12° corresponded to the heat which passed through 

 the glass. But when the alcohol flame, by its immediate action 

 upon the thermoscope, had produced a similar deviation of 20°, 

 the needle receded to 11° when the same glass plate was inserted 

 at the same spot ; consequently the heat of the alcohol flame 

 passed by radiation through the glass plate to a less extent than 

 that of the red-hot platinum. The heat of the Argand lamp, 

 which had also directly caused a deviation in the needle to 20°, 

 on inserting the glass produced a deviation of 15°. Finally, 

 when the hydrogen flame radiated upon the thermal pile so as 

 to deflect the needle to 20°, on inserting the glass screen it 

 returned to 12°. Hence it is evident that the heat of the hy- 

 drogen flame and the red-hot platinum, notwithstanding the 

 great difference in their temperature, is capable of passing through 

 a glass plate 1*3 millim. in thickness to an equal extent, but that 

 the heat of the alcohol flame possesses this power in a less degree 

 than that of the red-hot platinum, although its temperature is 

 higher than that of the latter, and the heat of the Argand lamp 

 in a much greater degree than that of the hydrogen flame, not- 

 withstanding its temperature is decidedly lower*. 



When, with the same direct action of the sources of heat, the 

 glass screen was exchanged for a plate of alum 1*4 millim. in 

 thickness, with red-hot platinum the magnetic needle receded to 

 8°'25 ; with the flame of alcohol, to 7°*5 ; the Argand lamp, to 

 10°'5 ; and with the flame of hydrogen, to 7°*75. 



Thus the heat of the hydrogen and alcohol flame, with great 

 difference in the temperature, passes through the plate of alum 

 to the same extent ; and that of the Argand lamp, and even that 

 of the red-hot platinum, pass through this more copiously than 

 the heat of the hydrogen flame, although they have a far less 

 degree of heat. 



* [This agrees exactly with Prof. Powell's results, Phil. Trans. 1825; and 

 according to that author's views the reason is obvious. — Ed.] 



