and the Mode of its Communication. 55 



metallic surfaces, those coverings must, by some means 

 or other, have facilitated and accelerated the emission 

 of calorific rays from the hot surface. 



Those suspicions implied, it is true, the supposition 

 that different substances, heated to the same tempera- 

 ture, emit unequal quantities of calorific rays ; but I 

 saw no reason why this might not be the case in fact ; 

 and I hastened to make the following experiments, 

 which put the matter beyond all doubt. 



Experiment No. 12. Two equal cylindrical vessels, 

 made of sheet brass, and polished very bright, each 

 3 inches in diameter and 4 inches long, suspended by 

 their oblique necks in a horizontal position (being 

 placed on their wooden stands), were filled with water at 

 the temperature of 1 80 ; and their circular flat bottoms 

 were presented in a vertical position to the two balls 

 of the thermoscope, at the distance of 2 inches. 



When the two hot bodies were presented, at the same 

 moment, to the two balls of the instrument, or, what 

 was still better, when two screens were placed before 

 the two balls, at the distance of about an inch, and, 

 after the hot bodies were placed, these screens were 

 both removed at the same instant, the small column of 

 spirit of wine, which I have called a bubble, remained 

 immovable in its place, in the middle of the horizontal 

 part of the tube of the instrument. 



If one of the hot bodies was now brought nearer the 

 ball to which it was presented (the other hot body re- 

 maining in its place), the bubble immediately began to 

 move from the hot body which was advanced forward, 

 towards the opposite ball to which the other hot body 

 was presented. 



If, instead of advancing one of the hot bodies nearer 



