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XXXIX. On Sensitive Flames. By W. F. Barrett, Lecturer 

 on Physical Science at the International College*. 



SINCE the publication of my "Note on Sensitive Flames" 

 in the last Number of the Philosophical Magazine, I 

 have arrived at what I believe to be the cause of the phe- 

 nomena there described. In that paper I showed that a tall 

 and voluminous flame of coal-gas, burning in the ordinary way, 

 under the influence of a shrill sound sank nearly half its height 

 and changed its shape, at the same time entering into a state 

 of rapid isochronous vibration which continued so long as 

 the sound was sustained. Hitherto the only explanation to this 

 effect is that given by Dr. Tyndall in his exposition of the sub- 

 ject at the Royal Institution, which is based on the fact that an 

 increased pressure similarly changes a sensitive flame. The ex- 

 planation given in the abstract of that lecture is, that "an external 

 sound added to that of a gas-jet already on the point of roaring 

 is equivalent to an augmentation of pressure on the issuing 

 stream of gas"f. But I venture to think that this statement 

 does not show how sonorous vibrations can effect an increase in 

 the pressure of the gas, nor does it explain the many perplexing 

 facts that have been observed. 



These perplexities are removed and the whole phenomena, it 

 appears to me, made very simple by the following considerations. 

 A sensitive flame is one which, on the slightest mechanical in- 

 crease in the pressure or, what here comes to the same thing, in 

 the velocity of the gas as it issues from the burner, will change its 

 shape and take very much the appearance it has when influenced 

 by sound. Now the sonorous pulses excited by a sound throw, 

 among other things, the pipe which conveys the gas to the 

 burner into vibration, the flow of gas is thereby driven from the 

 sides and urged moreto wards the centre of the tube ; and the 

 current, thus confined within narrower limits, must issue from the 

 burner with an increased velocity so long as the sound continues. 

 It is the greater rapidity thus induced in the issuing stream of 

 gas which causes the flame to shorten and diverge ; the lowering 

 of the flame being an analogous effect to that noticed and ex- 

 plained by Dr. Thomas Young in his well-known experiments on 

 streams of smoke escaping into the air at different velocities J. 



If the above explanation be correct, then certain facts should 

 be observed. For example, under the influence of sound the 

 gas, as it passes through the pipe leading to the burner, will 

 partake of the vibratory motion impressed on that pipe, the 



* Communicated by the Author. 



t Phil. Mag. February 1867, p. 99. 



% Phil. Trans. 1800, p. 112; Miller's < Chemical Physics,' p. 304. 



