THE ATMOSPHERE AND FOG-SIGNALING. 559 



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 recom- 

 menced. 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 de- 

 grees 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 respon- 

 sive 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. 



