262 Prof. Tyndall on Recent Researches on Radiant Heat. 



to the galvanometer. The receiver is first exhausted and the 

 screen removed; the consequent deflection gives the amount of 

 heat radiated against the pile through a vacuum. Air, or some 

 other gas, is then admitted, and the reduction of the deflection 

 is regarded as due to the absorption of the gas*. 



Air at the common laboratory temperature is here admitted 

 into direct contact with the radiating source possessing a tempe- 

 rature of 100° C; chilling of that source is the immediate con- 

 sequence. And no matter how long the gas may remain there, 

 the hot surface can never attain its pristine temperature. Prof. 

 Magnus, it will be observed, experiments in the ordinary way, 

 making use of one face only of his pile. I entirely failed to ob- 

 tain any absorption by air or any of the elementary gases by this 

 mode of experiment, while Prof. Magnus obtains for oxygen and 

 air an absorption of 11 per cent., and for hydrogen an absorp- 

 tion of 14 per cent. My apparatus enables me to measure an 

 absorption of 0*1 per cent. ; and surely with it an action so gross 

 as the above could never have escaped me. Nor could it have 

 escaped Melloni, who operated upon a column of air fifteen times 

 the length of that used by Prof. Magnus, and still found no ab- 

 sorption. With a column of air more than double the length of 

 his I obtain for oxygen only T y - th of the absorption ascribed to 

 it by Prof. Magnus, and only -riuth of what he finds for hydrogen. 



The greater action of hydrogen is quite in accordance with 

 the known chilling-power of that gas. While ascribing their 

 results to a different cause, some experiments of my own, which 

 I have briefly described in the paper recently presented to the 

 Royal Society, completely corroborate those of Prof. Magnus. 

 In these experiments the gases were allowed to come into direct 

 contact with the radiating source, and here the action of hydro- 

 gen bore to that of oxygen the precise ratio found by Prof. Mag- 

 nus. The tube used in these experiments was 8 inches long ; 

 and had I been tempted to ascribe the results to absorption, I 

 should have found in a tube of the above length fifty times the 

 effect observed in a tube 33 inches long, in which the gases were 

 withdrawn from contact with the source. 



The negative results of Prof. Magnus, as regards aqueous 

 vapour, are now sufficiently intelligible. The action which he 

 observed in the case of air being due to direct chilling by con- 

 tact — a process in which the mass of the chilling agent is the 

 most important consideration — the action of the minute quantity 

 of aqueous vapour present in the air becomes a vanishing quan- 

 tity. He makes air more than a hundred times what it ought 

 to be, and the action of the vapour practically disappears. 



It is curious and instructive to observe the contrast of opinion 

 between Prof. Magnus and myself. He concludes that even if 



