46 RADIATION EQUAL TO ABSORPTION. PAKT i. 



very simple arrangement. The thermo-electric pile 

 was raised on a stand with a screen of polished tin in 

 front of it. A heated copper ball in a perforated ring 

 on a low stand was placed behind the screen ; all direct 

 radiation from the ball was thus cut off, but the heated 

 air rising in a column above the screen radiated its heat 

 on the pile and deflected the needle of the goniometer 

 60 when the ball was red-hot ; but the radiation of the 

 hot air was neutralized by another source of radiant 

 heat on the opposite side of the pile which kept the 

 needle steadily at zero. Then a purified gas or vapour 

 conveyed by a pipe into the perforated ring which held 

 the ball rose mixed with the heated air above the screen, 

 but the radiation of the gas or vapour alone was shown 

 by the deflection of the needle, because that of the air 

 was compensated. With this apparatus Professor 

 Tyndall proved that the amount of the absorption of 

 each gas and vapour is exactly equal to the amount of 

 its radiation. He has shown that this result is a neces- 

 sary consequence of the dynamical nature of heat. For 

 as no atom or molecule is capable of existing in vibra- 

 ting ether without accepting a portion of the motion, the 

 very same quality whatever it may be that enables it to 

 do so, must enable it to impart its motion to still ether 

 when plunged into it. e Hence from the existence of 

 absorption we may on theoretic grounds infallibly infer 

 a capacity for radiation ; from the existence of radia- 

 tion we may with equal certainty infer a capacity for 

 absorption, and each of them must be regarded as the 

 measure of the other.' This reasoning, founded simply 

 on the mechanical relations of the ether and the atoms 

 immersed in it, is completely verified by experiment. 



Hitherto the absorption and radiation of heat by the 

 same thickness of different gases and vapours have been 

 compared with each other, but in a recent series of ex- 

 periments Mr. Tyndall has compared the action of dif- 



