200 LECTURES TO SCIENCE TEACHERS. 



gas is thus obtained, because there is no such cock here, 

 but a completely open way right to the burner. 



The source of sound is also important. I have here a 

 ticking watch inclosed in a padded case. I will wind up the 

 watch, and you will find how sensitive this flame is by its 

 being able to respond to the ticking of the watch. The 

 metallic sound being very rich in over tones or upper partial 

 tones, this sound is one to which the flame is particularly 

 sensitive, inasmuch as those notes are contained in the 

 sound made when the flame roars. Further it is the 

 root portion and not the upper portion of the flame which 

 is most sensitive. 



Now there remains to me to point out to you a few ap- 

 plications of these sensitive flames. You may use them in 

 detecting the existence of sonorous vibration in the air, or 

 the state of vibration of a sounding body. It is very in- 

 structive to examine, by means of a sensitive flame, the 

 vibrations of plates and bells, and so on. You find that 

 you can discover the nodal lines with great sharpness by 

 such means. Here I have an arrangement which 

 can be used as a practical application of these flames, in 

 the detection of sounds, such as a burglar filing his 

 entrance into a jeweller's shop, or it may be used in 

 automatically registering the presence of sonorous vibra- 

 tions. This little table supports three tubes, one of which 

 carries the flame. One rod carries a platinum wire, and 

 the other a compound riband of silver and platinum. 

 When this compound riband is warmed by the flame, it 

 bends and comes into contact with the platinum wire, and 

 thus closes a battery circuit and rings an electric bell. 

 Every time, therefore, the flame is under the influence of 

 sound, it will shorten and spread out sideways, and thus 

 touch the compound riband, and thereby closes the circuit 

 and rings the bell. I have usually a glass cylinder 

 surrounding this apparatus to keep it from the surrounding 

 draughts. 



It is extremely difficult in a class to show the ordinary 

 laws of reflection and refraction of sound, but a sensitive 

 flame enables us to do so with great ease. 



