122 Inquiry concerning tfie Nature of Heat, 



longing to that instrument ; consequently the instru- 

 ment will neither be heated nor cooled by these inter- 

 changes, but will continue invariably at the same con- 

 stant temperature. 



This explanation is plausible, but, before the hypoth- 

 esis on which it is founded can be admitted, we must 

 see if it will agree with the results of other experi- 

 ments, for the greatest care ought always to be used in 

 the admission of hypotheses in physical researches, and 

 in no case can it be more indispensably necessary than 

 where an hypothesis has evidently been contrived for 

 the sole purpose of explaining a single experiment, or 

 elucidating a new fact. 



When the surface of the metallic disk B was black- 

 ened by holding it over the flame of a candle, the in- 

 tensity of its radiation at the given temperature (that 

 of 112) was found to be very considerably increased; 

 and when (being so blackened) it was again presented 

 to the ball of the thermoscope at- the same distance as 

 in the last-mentioned experiment, and the cold disk A 

 (at the temperature of 32) was placed opposite to it at 

 an equal distance, as represented in Fig. 5, the thermo- 

 scope, instead of continuing to retain its original tem- 

 perature (that of 72), was now gradually heated. 



There is nothing, it is true, in that event, which ap- 

 pears difficult to explain on the assumed principles; for, 

 if the quantity of radiant caloric emitted by the disk B 

 be increased by blackening its surface, the quantity re- 

 ceived from it by the ball of the thermoscope must be 

 increased also, and that additional quantity must, of 

 course, tend to raise the temperature of the instrument. 

 But here is an experiment which cannot be explained on 

 those principles. 



