32 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
the galvanometer neutralized in the usual manner. The needle being at 0°, a film of 
olefiant gas, issuing from a narrow slit, was passed over the metal. The increase of 
radiation produced a deflection of 45°. When the gas was cut otf, the needle returned 
accurately to 0°. 
The absorption by a film may be shown by filling the cube with cold water, but not 
so cold as to produce the precipitation of the aqueous vapour of the atmosphere. A 
gilt copper ball, cooled in a freezing mixture, was placed in front of the pile, and its 
effect was neutralized by presenting a beaker containing a little iced water to the oppo- 
site face of the pile. A film of olefiant gas was sent over the ball, but the consequent 
deflection proved that the absorption, instead of being greater, was less than before. 
The ball, in fact, had been coated by a crust of ice, which is one of the best absorbers 
of radiant heat. The olefiant gas, being warmer than the ice, partially neutralized its 
absorption. When, however, the temperature of the ball was only a few degrees lower 
than that of the atmosphere, and its surface quite dry, the film of gas was found to act 
as a film of varnish ; it augmented the absorption. 
A remarkable effect, which contributed at first to the complexity 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 conse- 
quent 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 deflection greater. 
But the following action is really observed: — when the air first enters, the needle, 
instead of ascending, descends; 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 following 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 con- 
nected with a galvanometer. When air was admitted, a deflection was produced, 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 subse- 
quently had the tube perforated and thermometers screwed into it air-tight. On filling 
the tube the thermometric columns rose, on exhausting it they sank, the range between 
the maximum and minimum amounting in the case of air to 5° Fahe. 
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 
