64 Inquiry concerning the Natiire of 



a cold metal would or would not increase, in like man- 

 ner, 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 rilled 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 pre- 

 sented 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 conclu- 

 sive : 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 con- 

 stituted the index of the instrument, constantly show- 

 ing 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 



