PROFESSOR H. B. DIXOR' OX THE RATE OF EXPLOSION IN GASES. 107 
measurements of the rate were made at pressures varying from 200 mm, to 1500 mm. 
at a temperature close to 10" C. In the experiments at low pressures it was, of 
course, essential that the firing tube should be absolutely gas-tight; the tube was 
therefore tested before each experiment. An important fact concerning the develop¬ 
ment of the explosion-wave was detected in these experiments. As the pressure was 
reduced the flame was found to travel a greater distance before its rate became con¬ 
stant ; so that, although four feet start was allowed before the first measurement was 
taken, this distance was found insufficient. Accordingly a tube 20 feet long was 
interposed between the firing jDoint and the first bridge, after it had been shown that 
the flame acquired its maximum velocity in this distance under the lowest pressure 
used. Electrolytic gas can be exploded under lower pressures than 200 mm., but the 
flame travels irregularly. At 100 mm. pressure the flame went out in the tube, and 
at 150 mm. the rates found were not constant. 
Table VI.—Pressure Experiments. II3 + 0. 
Pressure. 
200 mm. 
300 mm. 
500 mm. 
760 mm. 
1100 mm. 
1500 mm. 
Mean rate 
2627 
2705 
2775 
2821 
2856 
2872 
These figures show that the rate of explosion increases rapidly wdth increase of 
pressure from 200 mm.; that the rate of increase diminishes, and that the velocity 
becomes nearly constant at two atmospheres pressure. This effect of pressure on the 
rate is plainly seen when the results are expressed graphically (see Plate 1). 
Berthelot’s conclusion that the explosion-wave is independent of the initial pres¬ 
sure of the gases is, therefore, not strictly accurate. At lower pressures the rate 
falls off*, but above a certain crucial pressure, which, in the case of hydrogen and 
oxygen, seems to be about two atmospheres, the velocity is independent of the 
pressure. 
It will be observed that the mean rate of explosion of hydrogen and oxygen at 
1500 mm. pressure is appreciably higher than Berthelot’s 9 —the calculated rate of 
translation of the steam molecules. But Berthelot, in his calculation, has not taken 
into account the fact that the gases are not at absolute zero to start with. Assuming 
that he worked at about 13° C., the theoretical rate would be 2900 metres per 
second. 
Analogous results were found in experiments with other mixtures under varying 
pressures. With hydrogen and nitrous oxide the rate is slower at 500 mm. than at 
760 mm., but no increase is found on raising the pressure to 1000 mm. The crucial 
pressure is in this instance below the atmospheric pressure. The same was found to 
be the case with marsh gas, ethylene, acetylene, and cyanogen exploded with oxygen. 
p 2 
