PROFESSOR H. B. DIXON ON THE RA.TE OF EXPLOSION IN GASES. 
149 
Le Chatelier found ‘‘ effective pressures ” in agreement with those obtained by 
Bunsen and by Berthelot, i.e., about 10 atmospheres. 
It is evident from these experiments that sudden pressures are produced in the 
explosion-wave greater than those shown by the experiments of Bunsen and of 
Berthelot. If the explosion is propagated by compression after the manner of a 
sound-wave enormously high temperatures must be produced in the wave, as I have 
already shown, and correspondingly high pressures must also exist. This aspect of 
the matter has been well stated by Mallard and Le Chatelier :— 
“ A gas, increasing in volume by combustion, exerts a certain pressure on the 
unburnt gas next to it : it is possible to conceive that this pressure may be raised 
sufficiently high to bring the neighbouring layer to the temperature of ignition. The 
possibility of the propagation of the inflammation by the propagation of the pressure 
can thus be understood. 
“ Take for example the explosive mixture of hydrogen and oxygen. According to 
our experiments it inflames at 555°, and the pressure exerted after the combustion in 
a closed vessel is 10 atmospheres. An infinitely thin layer inflamed by heating, 
reaches this pressure at the greatest, and cannot exert any higher pressure on the 
uninflamed layer next to it. 
“A compression of 10 atmospheres evolves a quantity of heat which can readily be 
calculated by thermo-dynamics : it is not sufficient to produce an elevation of tempe¬ 
rature equal to that inflammation. To obtain this temperature a compression of 
30 atmospheres would be needed. It would appear, then, that the explosion-wave 
could not be initiated under these conditions. 
“ Suppose, however, that the inflammation of the first layer is not brought about 
by heating, but by a suitable pressure, i.e., one of not less than thirty atmospheres, 
which may readily be produced by the detonation of a little fulminate. After the 
combustion the pressure of the inflamed layer will be multiplied by ten, or at least 
wiU be far higher than the initial pressure of thirty atmospheres, and will be able, 
consequently, to exert on the next layer a pressure sufficient to inflame it. This will 
continue then to propagate itself from layer to layer by the same mechanism.”— 
(‘ Combustion des Melanges Gazeux,’ p. 88.) 
The sudden state of pressure produced in each layer of burning gas ceases as 
suddenly as it is caused, by the expansion of the gas which compresses the unburnt 
layer in front; the high temperature of the burnt layer, thei’efore, rapidly falls by the 
transformation of heat into work. Mallard and Le Chatelier have shown by 
photographs of the flame that, in the explosion-wave, the luminosity of each layer does 
not last longer than one thousandth })art of a second ; while in the regime of ordinary 
combustion each layer remains luminous more than ten times as long. I can fully 
confirm these statements. By photographing the explosion-wave of hydrogen and 
oxygen on a fixed and on a rapidly moving plate at the same time, I have found that 
