350 



scribed show the operation of this law under conditions which 

 enable us more satisfactorily to mark the origin of the sound and 

 the gradations by which it accompanies the formation of the ex- 

 plosive mixture. 



(4.) The intermitting character of the combustion in a singing 

 flame has been beautifully shown by Prof Tyndal, by causing 

 the light of the flame to fall upon a revolving mirror, from which 

 it is reflected so as to form a series of images on a distant screen. 

 A similar resolution of the flame into successive explosions is 

 more simply exhibited by moving it rapidly to and fro, or better 

 still, by giving it a steady revolving motion within the tube. In 

 using the former method, the jet pipe may be attached to the 

 common gas stand by a short piece of flexible hose, and passed 

 through a ring so placed as to restrict the vibrations to a range 

 less than the diameter of the tube. A sufficiently regular move- 

 ment may then be given by the hand. If now, we adjust the 

 flame in the tube, so that it will not begin to sing for some time 

 after the vibration has commenced, or until the tube is further 

 lowered, we observe at first merely the continuous band of light 

 due to the permanence of the visual impression, but, as soon as 

 the singing commences, this band becomes waved or serrated at 

 the top, and with a proper velocity divides into nearly separate 

 columns of flame, with obscure spaces between. 



The effect is, however, far more striking, when the Jlame is 

 made to revolve at a uniform rate in the tube. In this case, so 

 long as it remains silent, it presents the appearance of a hollow 

 cylinder or short tube of whitish light ; but the moment the sing- 

 ing begins, the cylinder assumes a toothed form on the top 

 resembling a brilliant crown, and divides itself into a number of 

 narrow luminous columns, separated by bands nearly or quite de- 

 prived of light. It is hardly necessary to say that the obscure 

 spaces mark the moments in the rotation when the explosions 

 occur, and the bright ones the successive intervals between them. 

 With a given rate of rotation, as might be expected, the number 

 of these subdivisions is greater in a short tube than in a long 

 one, and is greater when the tube is yielding one of its hai'monic 

 notes than when giving its fundamental sound. In the same 

 tube the number of subdivisions diminishes as we increase the 

 velocity of rotation, a less number of vibrations or explosions in 

 this case corresponding to one revolution of the jet. 



