344 
PROFESSOR H. B. DIXON ON THE MOVEMENTS 
at C, where a “ return’’-wave is propagated hackwards. The return-wave is reflected 
at E and meets the flame (which has slightly receded) at D. The flame then 
advances rapidly to the end of the tube and sends back a retonation-wave from G. 
Now in this mixture a sound-wave travels about 300 metres per second. If a sound¬ 
wave started from A at the same time as the flame, it would reach the end of the 
tube at B and be reflected before meeting the advancing flame. On constructing the 
figure this reflected sound-wave was found to hit the point C. 
In another experiment the mixture C..No + Oo was fired at one end of a tube 
8 ^ inches lo:ig (215 millims.). No slackening of the flame was found, and no return- 
waves are visible. A very intense retonation-wave was throwii back from the end of 
the tube. On setting out the sound-wave from the firing j^oint we find that the 
sound is overtaken before it reaches the end of the tube (fig. 59.) 
In fig. 60 we see the effect of firing the mixture CoNg + Og 4 inches from one end 
of a tube 13 inches long. Fig. 60 a shows the outline of the visible waves and the 
path of the two sound-waves. The sound starting from A reaches the near end of 
the tube just before the flame; its reflexion coalesces with the intense retonation- 
wave. The sound-wave marching to the left is overtaken about 5^ inches from the 
spark. In fig. 61 the same mixture is fired in the centre of a tube 8 inches long. It 
confirms the result obtained in fig. 60 ; two intense retonation-waves are started 
simultaneously at either end. From these experiments we should infer that, if this 
mixture were fired at a point less than 4 inches from the end, the flame would be 
checked and return-waves would become visible. Fig. 62 shows this mixture fii’ed in 
the centre of a tube 6 inches long. The flame is checked symmetrically and the 
sound-waves produced cross and recross with great intricacy. 
With the less rapid mixture CgNg -f- 20g some experiments were made by firing 
the gas at the extreme end of the tube. A sound-wave starting at the firing point 
and travelling at the rate of 312 metres per second would be overtaken by the flame 
just before reaching the end of the tube, which was 12 inches long (fig. 63). On the 
other hand, when the tube was shorter the flame is checked by the returning sound¬ 
wave, which becomes visible as it traverses the incandescent gases (fig. 64). When 
the same mixture was fired in the centre of a tube 12 inches long, the sound-waves 
reach the two ends first, and produce symmetrical reflexions (fig. 65). 
These measurements afford, I think, conclusive evidence that compression-waves 
advance in front of the flame at the beginning of the explosion. AVhen the mixture 
is fired the gases, as they ignite, expand and send a series of pulses through the 
unhurnt gas, driving the molecules at every pulse bodily forward, and so increasing 
the pressure in the column of gas ahead. A¥hen the firing point is near the end 
of the tube, the head of this compression-wave returns and meets the advancing 
flame, and, of course, is propagated with increased velocity through the ignited gases. 
The return of the compression-wave, of course, checks and may reverse the bodily 
forward movement of the molecules in the flame, and thus oscillations are set up. 
