H 



232 KNOBLAUCH ON' RADIANT HEAT. 



In the present case it all depends upon our satisfactorily con- 

 vincing ourselves of the share taken by the latter. For this pur- 

 pose the substances introduced were blackened on the side next 

 the thermoscope, and thus the ti-ansmission, if any occurred, 

 was prevented. The differences which occurred on inserting 

 the body vuider examination, could now only be ascribed to its 

 becoming unequally heated. If it were adiathermanous, these 

 differences, even when the coating of lamp-black is removed, 

 remain the same, as we have previously (p. 220, 221) seen in a 

 thick layer of carmine and with black paper. If however it is 

 diatherrnanoiis, then by the access of the transmitted heat various 

 kinds of changes occur, \\ hich ai'e best illustrated by the expe- 

 riments themselves. 



AVhen the needle of the galvanometer was deflected to 40° by 

 the direct action of the source of heat, on inserting the black 

 glass coated with lamp-black next the thermal pile, in conse- 

 quence of its becoming heated, it stood at 12° when the Argand 

 lamp, and at 11° when the dark cylinder radiated upon it; 

 whilst in the first case it deviated to 16°'25, when the coating of 

 the lamp-black was removed, the deflection in the second case 

 remaining the same. Thus the difference in the indications with 

 the different sources of heat, amounted in blackened glass to 1°, 

 in that uncoated to 5°'25. This is an infallible proof, that on 

 removing the layer of lamp-black transmission occurs, which 

 acts in the same manner as absorption with regard to difference 

 of the deflections. 



On introducing the black lac, the different sources of heat pro- 

 duced an equal action upon the instrument as long as it was 

 blackened upon the side next the latter, but a different one when 

 the coating was removed. This difference however can only be 

 attributed to the transmission of the heat which is now per- 

 mitted. 



When the thin layer of carmine coated with lamp-black was 

 inserted, the needle receded from 40° to 17°'37 when the former 

 was exposed to the rays of the lamp, and to 21°"25 when ex- 

 posed to those of the metallic cylinder. On removing the coating 

 of lamp-black, it remained the first time at 19°*63, and the 

 second at 19°*87. The difference previously observed, arising 

 from the unequal heating, now disappears by a compensation of 

 transmission and absorption ; this being less in the case of the 

 heat of the flame than in that of the heated cylinder, the former 



