148 Royal Institution : — 



permitted to issue from the burner, strike the copper ball, and ascend 

 in a heated column in front of the pile. The result was that oxygen 

 showed itself, as a radiator of heat, to be quite as feeble as atmo- 

 spheric air. 



A second holder containing olefiant gas was also connected by 

 its own system of tubes with the ring-burner. Oxygen had already 

 flowed over the ball and cooled it in some degree. Hence, as a 

 radiator in comparison with oxygen, the olefiant gas laboured under a 

 disadvantage. It was purposely arranged that this should be the 

 case ; so that if, notwithstanding its being less hot, the olefiant gas 

 showed itself a better radiator, its claim to superiority in this respect 

 would be decisively proved. On permitting the gas to issue upwards, 

 it cast an amount of heat against the adjacent face of the pile sufficient 

 to impel the needle of the galvanometer almost to its stops at 90°. 

 This experiment proved the vast difference between two equally 

 transparent gases with regard to their power of emitting radiant 

 heat. 



The converse experiment was now performed. The thermo-electric 

 pile was removed and placed between two cubes filled with water kept 

 in a state of constant ebullition; and it was so arranged that the quan- 

 tities of heat falling from the cubes on the opposite faces of the pile 

 were exactly equal, thus neutralizing each other. The needle of the 

 galvanometer being at zero, a sheet of oxygen gas was caused to issue 

 from a slit between one of the cubes and the adjacent face of the pile. 

 If this sheet of gas possessed any sensible power of intercepting the 

 thermal rays from the cube, one face of the pile being deprived of the 

 heat thus intercepted, a difference of temperature between its two 

 faces would instantly set in, and the result would be declared by the 

 galvanometer. The quantity absorbed by the oxygen under those 

 circumstances was too feeble to affect the galvanometer ; the gas, in 

 fact, proved sensibly transparent to the rays of heat. It had but a 

 feeble power of radiation ; it had an equally feeble power of absorp- 

 tion. 



The pile remaining in its position, a sheet of olefiant gas was 

 caused to issue from the same slit as that through which the oxygen 

 had passed. No one present could see the gas; it was quite invisible, 

 the light went through it as freely as through oxygen or air, but its 

 effect upon the thermal rays emanating from the cube was what 

 might be expected from a sheet of metal. A quantity so large was 

 cut off that the needle of the galvanometer, promptly quitting the zero 

 line, moved with energy to its stops. Thus the olefiant gas, so light 

 and clear and pervious to luminous rays, was a most potent destroyer 

 of the rays emanating from an obscure source. The reciprocity of 

 action established in the case of oxygen comes out here ; the good 

 radiator is found by this experiment to be the good absorber. 



This result, which was exhibited before a public audience this 

 evening for the first time, was typical of what had been obtained with 

 gases generally. Going through the entire list of gases and vapours 

 in this way, we should find radiation and absorption to be as rigidly 

 associated as positive and negative in electricity, or as north and south 



