and the Mode of its Communication. 117 
a cold metal, would or would not increase, in like manner, the 
quantity of frigorific rays emitted by it. 
Having blackened, in the manner already described, the flat 
bottom, or rather end, of one of my horizontal cylindrical brass 
vessels with an oblique neck, I filled it with a mixture of ice 
and common salt ; and, filling another vessel of the same kind, 
the bottom of which was not blackened, with the same cold 
mixture, I presented them both, at the same instant, and at the 
same distance, to the two opposite balls of my thermoscope. 
The result of this experiment was perfectly conclusive : the 
bubble of spirit of wine began immediately to move towards the 
ball to which the blackened cold body was presented ; indicating 
thereby, that that ball was more cooled by the frigorific rays 
which proceeded from the blackened surface, than the opposite 
ball was cooled by the rays which proceeded from an equal 
surface of naked metal, at the same temperature. 
As this experiment appeared to me to be of great importance, 
I repeated it several times, and always with the same results ; 
the motion of the bubble, which constituted the index of the 
instrument, constantly showing that the frigorific rays from the 
blackened surface were more powerful, in generating cold, than 
those which proceeded from the naked metal. 
The bubble, it is true, did not move so far out of its place as 
it had done in the experiments in which hot bodies were presented 
to the balls ; but this was not to be expected ; for, though I had 
taken pains, by mixing salt with the ice, to produce as great a 
degree of cold as I conveniently could, yet still, the difference 
between the temperature of the balls and that of the bodies pre- 
sented to them, was much greater when the hot bodies were used, 
than when the experiments were made with the cold bodies; 
