could pierce the clear, optically-transparent, but acoustically-opaque atmosphere in the tunnel. So long as the gases continued to flow, the flame remained perfectly tranquil. When the supply was cut off, the gases rapidly diffused into the air. The atmosphere of the tunnel became again homogeneous, and therefore acoustically transparent, and the flame responded to each sound-pulse as before."
Not only do gases of different densities act thus upon sound, but atmospheric air in layers of different temperatures does the same. Across a tunnel resembling t t', Fig. 1, sixty-six platinum wires were stretched, all of them being in metallic connection. The bell, in its padded box, was placed at one end of the tunnel, and the sensitive flame k, near its flaring point, at the other. When the bell rang, the flame flared. A current from a strong voltaic battery being sent through the platinum wires, they became heated: layers of warm air rose from them through the tunnel, and immediately the agitation of the flame was stilled. On stopping the current, the agitation recommenced. In this experiment the platinum wires had not reached a red heat. Employing half the number and the same battery, they were raised to a red heat, the action in this case upon the sound-waves being also energetic. Employing one-third of the number of wires, and the same strength of battery, the wires were raised to a white heat. Here, also, the flame was immediately rendered tranquil by the stoppage of the sound.
But not only do gases of different densities, and air of different temperatures, act thus upon sound, but air saturated in different degrees with the vapors of volatile liquids can be shown by experiment to produce the same effect. Into the path pursued by the carbonic acid in our first experiment, a flask, which I have frequently employed to charge air with vapor, was introduced. Through a volatile liquid, partially filling the flask, air was forced into the tunnel t t' which was thus divided into spaces of air saturated with the vapor, and other spaces in the ordinary condition. The action of such a medium upon the sound-waves issuing from the bell is very energetic, instantly reducing the violently-agitated flame to stillness and steadiness. The removal of the heterogeneous medium restores the noisy flaring of the flame.
A few illustrations of the action of non-homogeneous atmospheres produced by the saturation of layers of air with the vapors of volatile liquids may follow here.
Bisulphide of Carbon.—Flame very sensitive, and noisily responsive to the sound. The action of the non-homogeneous atmosphere was prompt and strong, stilling the agitated flame.
Chloroform.—Flame still very sensitive; action similar to the last.
Iodide of Methyl.—Action prompt and energetic.
Amylene.—Very fine action; a short and violently-agitated flame was immediately rendered tall and quiescent.
Sulphuric Ether.—Action prompt and energetic.
