KXOBLAUCII ON RADIANT HEAT. 



387 



insertion was 13°. The repetitions of every three of these ob- 

 servations were made at intervals of five minutes only. 



3. The radiation yields the same values when (as shown in 

 subsequent experiments) one and the same reflecting substance 

 is used in different degrees of roughness, although in this case 

 unequal amounts of heat are absoi'bed and different amounts 

 would be radiated, if the water contained in the cubes did not 

 prevent this*. 



I therefore believe, that by the above process the absorption of 

 heat by the reflecting surfaces is sufficiently diminished to allow 

 of the assumption, that it has not perceptibly interfered with 

 the effects of the reflected heat. 



The constant fundamental deflection, which must be produced 

 by the heat reflected by the different bodies before the insertion 

 of the diathermanous media between the reflecting surface and 

 the therraoscope, might have been effected by appi'oximating or 

 removing the latter or the source of heat. But in both cases it 

 was impossible to protect the thermal pile from all external in- 

 fluences, and especially from the immediate action of the source 

 of heat during the reflexion. I therefore preferred producing 

 this deflection by a measured withdrawal and inclination of the 

 reflecting surface in regard to the instrument ; but, even then, 

 to be enabled really to judge of the changes which it has under- 



* These relations do not exist when the cube is exposed to the rays of lieat 

 witlivut water. The heat acquired by thcni then deflects tlie multiplier several 

 degrees, and causes a diminution of the variations after the insertion of the 

 diathermanous bodies in proportion to the extent of the share they have had in 

 the constant direct deflection. This is evident from the following examples : — 



Carmine .. 

 The same 

 ISIack lac 

 The same 



iMetal 



The same 



5 00 

 4-00 

 C-50 

 5-50 

 1000 

 10-25 



