206 



KNOBLAUCH ON RADIANT HEAT. 



3. That one and the same body is not uniformly heated by 

 rays of heat emanating from different sources, which exert the 

 same direct action upon a thermoscope coated with lamp-black 

 (see above, p. 188). I shall only select two very characteristic 

 obsei*vations from those which I have made in regard to this 

 point. 



When I coated a metallic disc on one side with carmine, on 

 the other with lamp-black, and exposed it immediately before 

 the thermal pile to the rays of an Argand lamp, in such a 

 manner that the carmine side was towards the source of heat 

 and the blackened one next the pile, it was found that when the 

 direct deflection of the multiplier-needle by the Argand lamp 

 amounted to 35°, that produced by the above arrangement 

 amounted to 9°"5. Under the same circumstances, however, I 

 obtained a deflection of 10°*l7j when, instead of a flame, I pro- 

 duced radiation upon the carmine-surface from a metallic cylin- 

 der at a heat below redness, which gave a direct deflection of 35°. 



When the metallic disc was covered with black paper instead 

 of carmine, the magnetic needle was deflected by the heat from 

 the plate placed before the thermal pile, in the first instance to 

 10° 75, in the second to 10°'12. 



Thus the carmine-surface is comparatively less heated by the 

 rays from the Argand lamp than by those of a cylinder heated 

 to 212° F., whilst with black paper exactly the contrary occurs. 

 The following numbers, which refer to a greater dii-ect deviation 

 (each being the arithmetical mean of two observations), will show 

 this still more decidedly : — 



Table VI. 



Whilst from former observations the rays of heat emitted by 

 sources having a lower temperature appear almost invariably to 

 be more capable of heating bodies than those at a higher tem- 

 perature, it is clearly shown by the experiments we have just 



