PROFESSOR H. B. DIXON ON THE RATE OP EXPLOSION IN GASES. 
131 
. . . The combustion of each layer produces both heat and at the same time the 
wo]‘k necessary to compress the following layer—that is to say, the layer loses on this 
score just as much heat as it gained by its own compression. The whole proceeds, as 
far as the elevation of temperature is concerned, precisely as if we had operated under 
constant pressure,” On the facts of the case there is no dispute. The gas is exploded 
in a closed vessel. Each layer is compressed before being fired; after firing it 
compresses the layer beyond it. Now, as regards this preliminary compression, each 
layer, in turn, expends the same energy as was previously communicated to it, and, 
therefore, it does no work of its own. But a gas heated under conditions where it 
does no work is raised to the same temperature as it would be had its volume 
remained constant. M. Berthelot admits that it would appear at first sight as if 
the gases were heated at constant volume ; he adds that the concordance of the 
calculated with the observed numbers supports his explanation of the phenomena. 
Again, the fact that each layer is fired by compression involves the preliminary 
heating of that layer, and this heating must be added to the heat developed by its 
burning. The temperature of the burning layer must, therefore, be greater than 
that obtained by dividing the heat developed in the chemical change by the specific 
heat of the products of combustion. 
When a sound-wave alone is transmitted through a gas its velocity shows that 
each layer of gas forming the wave-front is heated by compression, and there is no 
reason why this should not happen when the compression is accompanied by a 
chemical change. 
To what extent is each layer heated before combustion ? MM. Mallard and 
Le Chatelier* state that the explosion-wave will be propagated when each layer 
is brought by compression to its own temperature of inflammation. For hydrogen 
and oxygen, they find this temperature to be about 5.50° C., and calculate that a 
pressure of 30 atmospheres must be exerted upon the gas to raise it to this point. 
This temperature may be regarded as the lowest limit of the preliminary heating of 
each layer before combustion. But having regard to the fact that the individual 
molecules of hydrogen and oxygen in a jmt unburnt layer may, in the interval of time 
between their meetings, be heated above the point at which they combine with each 
other, we cannot affirm that this limit of temperature may not be greatly surpassed 
in the explosion. 
If we regard the transmission of the explosion to be due to the collisions of the 
molecules, and assume that molecules which are chemically inactive towards each 
other act as elastic bodies when they come into collision, and that molecules which 
combine chemically lose energy of translation and gain energy of vibration, then it 
must happen that unburnt molecules come into collision with burnt molecules and take 
up their energy in the form of motion. For instance, in the explosion of hydrogen 
and chlorine, the energy of the hydrogen chloride formed will be communicated by 
* ‘ RecLerclies sur la combustion des Melanges Gazeux Explosifs,’ p. 88-91. 
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