278 Prof. Tyndall on the Absorption and 



tion, while when the wire was withdrawn no trace of radiant 

 heat could be detected by his apparatus. He concluded from 

 this experiment that air possesses the power of radiation in so 

 feeble a degree, that our best thermoscopic instruments fail to 

 detect this power*. 



These are the only experiments hitherto published upon this 

 subject; and I have now to record those which have been made 

 in connexion with the present inquiry. The pile furnished with 

 its conical reflector was placed upon a stand, with a screen of 

 polished tin in front of it. An alcohol lamp was placed behind 

 the screen so that its flame was entirely hidden by the latter ; 

 on rising above the screen, the gaseous column radiated its heat 

 against the pile and produced a considerable deflection. The 

 same effect was produced when a candle or an ordinary jet of 

 gas was substituted for the alcohol lamp. 



The heated products of combustion acted on the pile in the 

 above experiments, but the radiation from pure air was easily 

 demonstrated by placing a heated iron spatula or metal sphere 

 behind the screen. A deflection was thus obtained which, 

 when the spatula was raised to a red heat, amounted to more 

 than sixty degrees. This action was due solely to the radiation 

 of the air; no radiation from the spatula to the pile was possible, 

 and no portion of the heated air itself approached the pile so as 

 to communicate its warmth by contact to the latter. These 

 effects are so easily produced that I am at a loss to account for the 

 inability of so excellent an experimenter as Melloni to obtain them. 



My next care was to examine whether different gases possessed 

 different powers of radiation ; and for this purpose the following 

 arrangement was devised. P (fig. 1) represents the thermo-electric 

 pile with its two conical reflectors ; S is a double screen of po- 

 lished tin j A is an argand burner consisting of two concentric 

 rings perforated with orifices for the escape of the gas ; C is a 

 heated copper ball ; the tube 1 1 leads to a gas-holder containing 

 the gas to be examined. When the ball C is placed on the 

 argand burner, it of course heats the air in contact with it ; an 

 ascending current is established, which acts on the pile as in 

 the experiments last described. It was found necessary to 

 neutralize this radiation from the heated air, and for this pur- 

 pose a large Leslie's cube L, filled with water a few degrees 

 above the temperature of the air, was allowed to act on the op- 

 posite face of the pile. 



When the needle was thus brought to zero, the cock of the 



gas-holder was turned on ; the gas passed through the burner, 



came into contact with the ball, and ascended afterwards in a 



heated column in front of the pile. The galvanometer was now 



* La Thermochrose, p. 94. 



