1874.] by a Layer of Flame or Heated Gas. 191 



was allowed to stream upwards across the end of the tin tube B A at A. 

 A portion of the sound-wave issuing from the tube was reflected at the 

 limiting surfaces of the heated layer; and a part being transmitted 

 through it, was now only competent to slightly agitate the sensitive 

 flame at F'. 



The heated layer was then placed at such an angle that the reflected 

 portion of the sound-wave was sent through a second tin tube, A E (of 

 the same dimensions as B A), its action being rendered visible by its 

 causing a second sensitive flame placed at the end of the tube at F to 

 become violently affected. This action continued so long as the heated 

 layer intervened ; but upon its withdrawal the sensitive flame placed at 

 F', receiving the whole of the direct pulse, became again violently agi- 

 tated, and at the same moment the sensitive flame at F, ceasing to be 

 affected, resumed its former tranquillity. 



Exactly the same action takes place when the luminous portion of a 

 gas-flame is made the reflecting layer ; but in the experiments above 

 described, the invisible layer above the flame only was used. By proper 

 adjustment of the pressure of the gas, the flame at F' can be rendered 

 so moderately sensitive to the direct sound-wave, that the portion trans- 

 mitted through the reflecting layer shall be incompetent to affect the 

 flame. Then by the introduction and withdrawal of the bat's-wing flame 

 the two sensitive flames can be rendered alternately quiescent and 

 strongly agitated. 



An illustration is here afforded of the perfect analogy between light 

 and sound ; for if a beam of light be projected from B to F', and a plate 

 of glass be introduced at A, in the exact position of the reflecting layer 

 of gas, the beam will be divided, and one portion will be reflected in the 

 direction A F, and the other portion transmitted through the glass in 

 the direction F', exactly as the sound-wave is divided into a reflected and 

 transmitted portion by the layer of heated gas or flame. 



