196 LECTURES TO SCIENCE TEACHERS. 



Various illustrations of this will at once occur to you. A 

 stone poised on the edge of a cliff, for example, can be 

 thrown off by a series of timed puffs puffs recurring at 

 the rate of vibration of the stone may cause its overthrow, 

 and thus a great change may be brought about. Prince 

 Rupert's drops exhibit this sensitive condition. Certain 

 fulminating powders also exhibit this sensitiveness. Or 

 again, if we take a singing flame, and get it nearly at the 

 point of singing, and then sound a note exactly similar to 

 that which the flame itself makes when singing, the flame 

 will be thrown into continuous song. 



Now a naked flame may respond without a tube. If 

 we take an ordinary gas burner such as I have here, 

 and bring it near to the point of roaring, then a sound 

 made near that flame will throw it into a roaring con- 

 dition, accompanied by a shortening of the flame. A slight 

 motion of the flame is perceived. This effect, however, 

 which is exceedingly slight, can be augmented by properly 

 adjusting the weights on the gasholder until we obtain a 

 sensitive flame, properly so called. Here I have such a 

 flame which consists of coal-gas burning from a narrow 

 orifice, yielding us a tall tapering flame which responds to 

 a very slight sound, such for example as the sound of the 

 sibilant. 



Now, regarding the history of sensitive flames, it will be 

 sufficient for me to say that the subject was first brought 

 prominently under public notice by Professor Tyndall, who 

 at the same time enriched the discovery that had pre- 

 viously been made. You probably see already the explan- 

 ation of this phenomenon. The flame accepts the vibration 

 which it can itself emit. If, for example, we light this 

 flame, and now put a little pressure upon the gasholder, 

 you will find that the flame will begin to shorten and roar, 

 just as it did under the influence of sound, making that 

 sibilant noise. A pressure of an extremely slight amount 

 will cause a flame which is on the verge of roaring, to roar. 

 Far less than one hundredth of an inch of water pressure 

 will cause an ordinary batswing burner to turn from a 

 silent flame into a roaring flame. We have, therefore, in a 

 sensitive flame a body in a state of sympathetic vibration, 

 and the vibration of the flame is exactly synchronous to 

 that of the sound which throws it into vibration. The 



