338 Prof. Tyndall on the Absorption and 



a thermometer plaeed within the tube at the end furthest from 

 the source, and one placed without it. I then examined, by an 

 extremely sensitive thermometer, the increase of temperature 

 produced by the admission of dry air into the tube, and the 

 decrease consequent on pumping out, and found the former to 

 be a considerable fraction of the total heat transmitted from the 

 source. Could it be that the heat thus imparted to the alcohol 

 and ether vapours, and radiated by them against the adjacent face 

 of the pile, was more than sufficient to make good the loss by 

 absorption ? The experimentum cruris at once suggested itself 

 here. If the effects observed be due to the dynamic heating of 

 the air, we ought to obtain them even when the sources of heat 

 made use of in the experiments are entirely abolished ; and we 

 should thus arrive at the solution of the novel and at first sight 

 utterly paradoxical problem, To determine the radiation and ab- 

 sorption of gases and vapours without any source of heat external 

 to the gaseous body itself. 



For the sake of brevity, I will call the heating of gas by its 

 admission into a vacuum, the dynamic heating of the gas ; and 

 the chilling accompanying its pumping out, dynamic chilling. 

 It would also contribute to brevity if I were allowed to call the 

 radiation and absorption of the gaseous body, consequent on such 

 heating and chilling, dynamic radiation and dynamic absorption, 

 though I fear the terms are not unobjectionable. 



§ 7. On Dynamic Radiation and Absorption. — Both the source 

 of heat and the compensating cube were dispensed with, and the 

 thermo-electric pile was presented to the end of the cold experi- 

 mental tube. By a little management, the slight inequality of 

 radiation against both faces of the pile, arising from differences 

 in the various parts of the laboratory, was obliterated, and the 

 needle of the galvanometer thus brought to 0°. 



The vapours were admitted in the manner already described, 

 until a tension of 0*5 of an inch was obtained. The air was then 

 allowed to enter through a drying-apparatus by an orifice of a 

 constant magnitude. Two stopcocks, in fact, were introduced 

 between the drying- tube and the experimental tube ; one of these 

 was kept partially turned on, and formed the gauge for the ad- 

 mission of the air. When the tube was to be exhausted, the 

 second stopcock was turned quite off. When the tube was to be 

 filled, this stopcock was turned full on ; but the gauge-cock was 

 never touched during the entire series of experiments. 



Before, however, the mode of experiment was thus strictly ar- 

 ranged, a few preliminary trials gave me the following results : — 



Nitrous oxide on entering caused the needle to swing in a 

 direction which indicated the heating of the gas ; the limit of 

 its excursion was 28°, after which it slowly sunk to 0°. 



The pump was now worked; the propulsion of the first por- 



