-174 C° un t Rumford's Enquiry concerning the Nature of Heat, 
at the given temperature, (that of 1 is 0 ,) was found to be 
very considerabiy 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 0 ) was placed opposite to it, at an 
equal distance, as represented in Fig. 5, the thermoscope, in- 
stead of continuing to retain its original temperature, (that of 
72 0 ,) was now gradually heated. 
There is nothing, it is true, in that event, which appears diffi- 
cult 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 received from it by the ball of the ther- 
moscope 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. 
The surface of the cold disk A having been blackened, as 
well as that of the hot disk B, when both disks (blackened) were 
again presented, at equal distances, to the ball of the thermo- 
scope, as represented in Fig. (S, it was found, that the original 
temperature of the thermoscope remained unchanged. 
The result of this most interesting experiment proves, that 
the ball of the thermoscope was just as much cooled by the 
influence of the cold blackened disk, as it was heated by the hot 
blackened disk. 
Now, as it was found by experiment, that the intensity of the 
radiation of the disk B was increased by the blackening of the 
surface of that disk, we must conclude, that the intensity of 
the radiation of the disk A was likewise increased by the use 
