220 KNOBLAUCH ON RADIANT HEAT. 



is turned towards both the thermal pile and the source of heat, 

 two things may happen. Either the above difference in the de- 

 flections remains constant in proportion to them : the radiating 

 power, even with the carmine, then has no share in it ; or it is 

 altered : when the unequal radiation of the carmine-surface is 

 proved to occur in both cases. If this, e.g. is increased, it 

 would be a proof that the carmine-surface radiates comparatively 

 better when heated by the metallic cylinder than when heated 

 by the Argand lamp, for the same reason, perhaps, because it 

 absorbs the former better than the latter. 



Experiment has decided in the first case ; for the blackened 

 carmine-surface placed next the pile produced a deflection of 9°'5 

 when exposed to the rays of the Argand lamp, and of 10°"87 

 when exposed to those of the heated cylinder. The difference 

 thus amounted to 1°'37. The free radiating carmine-surface in 

 the first case caused the same deflection in the needle of 9°*5, in 

 the second of lO'^'S. The difference of 1°'0 thus found with the 

 same intensity of the deflections, the first being 1°*37, is not 

 greater than comes within the limits of error of observation. 



The same occurred on using black paper. Thus when it was 

 coated with lamp-black on the side next the pile, a deflection of 

 10°-75 was produced by the rays of the Argand lamp, and of 

 10°"12 by those of the heated cyUnder; when the radiation was 

 free, in the case of the former a deflection of 10°'62, of the latter 

 of 9°*87. In the first experiment the difference was 0° 63, in 

 the second 0°*75. Both may be considered identical; and hence 

 we must conclude that the radiating power of the black paper is 

 independent of the nature of the heat absorbed. 



The following table, in addition to the observations detailed, 

 which refer to a direct deflection of 35°, contains others for a 

 greater intensity of the sources of heat; these also yield the 

 same results. (The numbers are each the arithmetic mean of 

 two observations.) 



