Radiation of Heat by Gases and Vapours. 281 



A remarkable effect, which contributed at first to the com- 

 plexity of the experiments, can now be explained. Conceive the 

 experimental tube exhausted and the needle at zero ; conceive a 

 small quantity of alcohol or ether vapour admitted ; it cuts off a 

 portion of the heat from one source, and the opposite source 

 triumphs. Let the consequent deflection be 45°. If dry air be 

 now admitted till the tube is filled, its effect of course will be 

 slightly to augment the absorption and make the above deflec- 

 tion greater. But the following action is really observed: — 

 when the air first enters, the needle, instead of ascending, de- 

 scends ; it falls to 26°, as if a portion of the heat originally cut 

 off had been restored. At 26°, however, the needle stops, turns, 

 moves quickly upwards, and takes up a permanent position a 

 little higher than 45°. Let the tube now be exhausted, the 

 withdrawal of the mixed air and vapour ought of course to 

 restore the equilibrium with which we started; but the follow- 

 ing effects are observed : — When the exhaustion commences, the 

 needle moves upwards from 45° to 54°; it then halts, turns, and 

 descends speedily to 0°, where it permanently remains. 



After many trials to account for the anomaly, I proceeded 

 thus : — A thermo-electric couple was soldered to the external 

 surface of the experimental tube, and its ends connected with a 

 galvanometer. When air was admitted, a deflection was pro- 

 duced, which showed that the air, on entering the vacuum, was 

 heated. On exhausting, the needle was also deflected, showing 

 that the interior of the tube was chilled. These are indeed 

 known effects; but I was desirous to make myself perfectly sure 

 of them. I subsequently had the tube perforated and thermo- 

 meters screwed into it air-tight. On filling the tube the ther- 

 mometric columns rose, on exhausting it they sank, the range 

 between the maximum and minimum amounting in the case of 

 air to 5° Fahr. 



Hence the following explanation of the above singular effects. 

 The absorptive power of the vapour referred to is very great, and 

 its radiative power is equally so. The heat generated by the air 

 on its entrance is communicated to the vapour, which thus 

 becomes a temporary source of radiant heat, and diminishes the 

 deflection produced in the first instance by its presence. The 

 reverse occurs when the tube is exhausted; the vapour is chilled, 

 its great absorptive action on the heat radiated from the adjacent 

 face of the pile comes more into play, and the original effect is 

 augmented. In both cases, however, the action is transient; 

 the vapour soon loses the heat communicated to it, and soon 

 gains the heat which it has lost, and matters then take their 

 normal course. 



§ 10. On the Physical Connexion of Radiation, Absorption, and 

 Phil. Mag. S. 4. Vol. 22. No. 147. Oct. 1861. U 



