330 FEAGMENTS OF SCIENCE. 



at zero. The rays emitted by the, current of hot air 

 already referred to were permitted to fall upon one of 

 the faces of the pile ; and an extremely slight move- 

 ment of the needle showed that the radiation from the 

 hot air, though sensible, was extremely feeble. Con- 

 nected with the ring-burner was a holder containing 

 oxygen gas ; and by turning- a cock, a stream of this 

 gas was 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 atmospheric air. 



A second holder containing olefiant gas was then 

 connected with the ring-burner. Oxygen and air had 

 already flowed over the ball and cooled it in some degree. 

 Hence the olefiant gas laboured under a disadvantage. 

 But on permitting the gas to rise from the ball, it casts 

 an amount of heat against the adjacent face of the pile 

 sufficient to impel the needle of the galvanometer almost 

 to 90. This experiment proved the vast difference 

 between two equally invisible 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 quantities 

 of heat falling from the cubes on the opposite faces of 

 the pile were exactly equal, thus neutralising 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 inter- 

 cepted, a difference of temperature between its two 

 faces would instantly set in, and the result would be 



