and thus loaded with thick fumes, was sent into the tunnel; the agitated flame was rendered immediately quiescent, indicating a very decided action on the part of the artificial fog.
Air passed through perchloride of tin and sent into the tunnel produced exceedingly dense fumes. The action of the fog upon the sound-waves was very strong.
The dense smoke of resin, burnt before the open end of the tunnel and blown into it with a pair of bellows, had also the effect of stopping the sound-waves, so as to still the agitated flame.
The result seems clear; and it perfectly harmonizes with the prevalent a priori notions as to the action of fog upon sound. But caution is here necessary; for the smoke of the brown paper was hot; the flask containing the hydrochloric acid was hot; that containing the perchloride of tin was hot; while the resin-fumes produced by a red-hot poker were also obviously hot. Were the results, then, due to the fumes or to the differences of temperature? The observations might well have proved a trap to an incautious reasoner.
Instead of the smoke and heated air, the heated air alone from four red-hot pokers was permitted to stream upward into the tunnel; the action on the sound-waves was very decided, though the tunnel was optically empty. The flame of a candle was placed at the tunnel-end, and the hot air just above its tip was blown into the tunnel; the action on the sensitive flame was decided. A similar effect was produced when the air, ascending from a red-hot iron, was blown into the tunnel.
In these latter cases the tunnel remained optically clear, while the same effect as that produced by the resin—smoke and fumes—was observed. Clearly, then, we are not entitled to ascribe, without further investigation, to the artificial fog an effect which may have been due to the air which accompanied it.
Having eliminated the fog and proved the non-homogeneous air effective, our reasoning will be completed by eliminating the heat, and proving the fog ineffective.
Instead of the tunnel a b c d (see p. 689), a cupboard with glass sides, three feet long, two feet wide, and about five feet high, was filled with fumes of various kinds. Here it was thought the fumes might remain long enough for differences of temperature to disappear. Two apertures were made in two opposite panes of glass three feet asunder; in front of one aperture was placed the bell in its padded box and behind the other aperture, and at some distance from it, the sensitive flame.
Phosphorus placed in a cup floating on water was ignited within the closed cupboard. The fumes were so dense that considerably less than the three feet traversed by the sound extinguished totally a bright candle-flame. At first there was a slight action upon the sound; but this rapidly vanished, the flame being affected exactly as if the sound
