114 PROFESSOR H. B. DIXON ON THE RATE OF EXPLOSION IN GASES. 
Some experiments published by Berthelot in 1885'" seem to be at variance with 
this statement. When electrolytic gas, largely diluted with oxygen, was exploded in 
a bomb, the flame traversed the bomb more quickly than when an equal volume of 
nitrogen was used as a diluent. But the rate measured in this case was not that of 
the explosion-wave ; it was the mean rate of the flame from the point of inflamma¬ 
tion to a point about 250 mm. distant. On the other hand, with a smaller volume of 
diluent gas, Berthelot found the rate of explosion to be less retarded by nitrogen 
than by oxygen :— 
Time taken by flame to travel 250 mm. 
Hg -h O.2 • 1 4 thousandths of a second. 
Ho + O + 3 O 2 . . . . 16-04 
Htj d“ O -j- 3 N 3 . . . 24'45 ,, ,, 
^2 + O fl- Oo . . . . 8-16 ,, ,, 
Ho + O + No. . . . 6-87 
I have found that the explosion-wave is sooner initiated when electrolytic gas is 
largely diluted with oxygen than when it is diluted with the same volume of 
nitrogen ; a fact which explains the apparent anomaly in Berthelot’s experiments. 
On repeating my measurements of the rate of explosion of electrolytic gas with 
large volumes of diluent gases, some discrepancies were found, which could not at 
first be explained. A tube of larger diameter (13 mm.) was then substituted for the 
smaller tube (9 mm.). The results were more regular. It was then discovered that 
the explosion frequently died out near the end of the smaller tube, whilst transmit¬ 
ting a sound-wave sufliciently powerful to break the silver foil composing the “ second 
bridge.” A record was thus obtained on the chronograph plate, indicating a smaller 
velocity than the true rate of explosion. By inserting a glass tube just before the 
second bridge, and watching for the flame, the observers made sure that the explosion- 
wave reached the end of the pipe. A second precaution, which must also be observed 
in experiments on largely diluted gases, is to make sure that the explosion-wave has 
been set up before the flame reaches the first bridge, M. Berthelot does not state 
the free run he allowed his flame before beginning the measurements; but, from an 
inspection of the figure of his apparatus (‘ Sur la Force des Matieres Explosives,' 
vol. 1, p. 138) it would appear that a few inches only were allowed bet-ween the 
firing spark and his first “ interrupter.” No doubt, the use of a grain of fulminate in 
the interrupter hastened the establishment of the explosion-wave in his experiments, 
but I tliink it possible that in some of his measurements he began to record the rate 
before the wave was started, and so obtained a rate below the true one. In all my 
later experiments on diluted gases, some pure electrolytic gas was j^assed into the long 
firing piece, just before the explosion, so as to fill it for a length of tw-o or three feet. 
* ‘ Ann. Cliiin. Phys.’ [VI.], vol. 4, p. 44. 
