PEOFESSOR H. B. DIXON ON THE RATE OF EXPLOSION IN GASES. 155 
to a forward movement an masse of the burning molecules, or to the advance of a 
comparatively few molecules with extreme rapidity. In the latter case the ex¬ 
plosion-wave might be propagated with a velocity far higher than the average 
velocity of the molecules of burning gas. I think the available evidence precludes 
the idea that the ignition can be propagated by a few molecules moving with 
velocities far above the mean. For, if this were the case, the presence of a diluent 
gas ought not to hinder the advance of the wave in the manner it is found to do. The 
heat evolved on the formation of a compound molecule is the same whether inert 
molecules are in tlie neighbourhood or not, and for the moment the temperature of 
the compound is the same. If, therefore, the ignition could be propagated by the 
advance of individual molecules, we might expect the explosion to be propagated at 
the same, or nearly the same, rate in the presence of an inert gas. My experiments 
have, however, conclusively shown that an inert gas retards the wave by an amount 
which depends upon its volume and density. It would seem to follow that each 
layer is heated as a whole and that the advance of the wave depends upon the 
average motion of the molecules. Two other facts appear to support this view. It 
is well known that small electric sparks may be sent through explosive mixtures of 
gases, e.g., through electrolytic gas, without causing explosion. Yet the temperature 
in the spark must be extremely high, and combination occurs in the path of the spark. 
It would seem that the number of molecules heated by each spark was not sufficient to 
communicate ignition to the rest. Again, when a sound-wave is started in a mixture 
of gases, its rate of propagation depends upon the mean density of the gaseous 
mixture; there is no selective propagation of the wave by one of the constituents of 
the mixture.* This fact, of which I could find no experimental demonstration for 
gases of widely different density, has been verified, at my request, by Mr. F. J. Smith 
in the Millard Laboratory, at Trinity College, Oxford. According to Mr. Smith’s 
careful experiments, sound is propagated at the same rate in ammonia gas (density 8 A) 
as in a mixture of equal volumes of hydrogen and oxygen of the same mean density as 
ammonia. 
On the other hand, it may be that the wave does not advance perfectly regularly, 
but breaks, like a water-wave is broken by the quicker movement of the crest. In 
breaking, jets of heated gas may be projected in front of the wave into the nnburnt 
gas beyond—such jets becoming the centres of fresh disturbances which coalesce to 
* Faraday thougEt that gases of very different density would not mix perfectly. “ As the atmosphere 
is now constituted, there exists a permanency of sonorous pitch; any tone being once generated remains 
the same tone until it dies away. . .' . If, however, the atmosphere had been composed of two gases, 
each having widely diffei’ent specific gravities, there would have been a difference. No permanency of 
tone could then have been depended upon—the pitch of every original note would have been continually 
varying as its tra*nsmission might be propagated by the vibrations of ever-varying mixtures of the two 
gases of dissimilar specific gravities.” Faraday’s Lectures, Scoffern, 1853. 
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